Underwater quest

[Mikalo]

So in the last quest the players discorevered that they are pieces of data in a system that is going to shut down soon, with the help of a A.I (a sort of god) they escape in a deep "sea" waiting to spawn in the new system. I wanted to make it a bunch of underwater puzzles the pc have to solve before running out of air, with "geysers" providing occasional oxygen bubbles. The goal is to open the door leading to the new system.
The party is level 4 and I have to keep in mind that there is a Druid, a Cleric and a very fat Crusader. They have no potions or magic items that can help.

Any idea to make it more fun? Like adding more threats or make the puzzles extremely hard (one of the players is a sort of math prodigy, I don't want him to ruin what I think is really clever >_>

[Endarire]

What about providing items of water breathing or an NPC caster of such?

[Mikalo]

Where is the fun if they can go around and chill? :(

[HammeredWharf]

Do your players like puzzles in their RPGs? Many don't.

I'd make a bunch of environmental hazards instead, the difference being that the players would be able to use their characters' abilities to escape. For example, there's a really fast stream you've got to pass here and a nasty shark is chasing you. Do you try to swim straight through it (Swim DC 20)? Do you study the rocky tunnel (K: Nature 15) to see an easier way through (Swim 15) or a weak point you can break to slow the stream down (Str 15, Swim 10)? Do you tame the shark and ride it through the stream (Wild Empathy 25, then Ride 10)?

The premise also gives you opportunity to make some surrealistic reality breakdowns, since it's the end of the universe. What does it look like? Does it pose unique challenges? Weird pockets of gravity? Pillars of water floating in nothingness? Invisible walls? A border patrol straight from Lovecraft's nightmares?

[Zaq]

Time limits of the sort you're describing are difficult to do in D&D.

I would imagine (perhaps I am making an unjustified assumption here, but hopefully not too unjustified) that you're thinking of the sort of level/area found in many video games where the player has a limited amount of time (sometimes extended with bubbles or other sources of air) to get through the area and/or complete their chosen goal before their character drowns, yes? That works decently in video games (how much fun it actually is, of course, is a secondary discussion—I personally am generally not fond of that sort of area) because the challenge comes down to the player's ability to skillfully control their character in the necessary manner under an immediate time limit. It puts an immediate time pressure on the player in a way that the rest of the game usually doesn't, basically raising the stakes for missed jumps or bad choices or whatever.

This is basically impossible to translate to D&D, because D&D doesn't (and almost always shouldn't) have a real-time element. Unless you have a group that's way, way more coordinated than any I've ever seen, putting even a relatively generous real-time time limit on solving a puzzle usually devolves into the group screaming at each other or simply panicking. Not only are the players not going to be used to a real-time time limit, but if that time limit is eaten up equally by deciding what to do (both individually and as a group), getting the GM's attention to narrate your own specific action as opposed to letting the GM resolve the other players' actions, resolving the action with whatever die rolls or rule lookups are necessary, letting the GM decide/narrate the success/failure/result/progress of your actions, and whatever other elements go into whatever happens at your table (maybe including things like asking the GM for description of the area), well, that honestly seems like there's a lot of ways that it could be unfair. It must also be remembered that on a round-by-round level, figuring out what a character does and what happens as a result takes far more time out-of-character than it does in-character. (When things aren't happening on a round-by-round basis, we can narrate things to go much faster than they would in real time, but that's not the case when we aren't glossing over things.) So I think that it's basically just going to be courting disaster to put a real-time time limit on things, especially if the players aren't expecting it ahead of time.

If you aren't putting a real-time time limit on things, though, then the pressure gets very different, and honestly, it does get kind of weird. The main pressure on the players is going to be making sure that their characters don't perform too many extraneous actions or take too many rounds doing things that aren't essential to getting out of the water alive. That might actually make things slow way the hell down, since the players (if they're anything like me or like the people I usually play with) are going to get super cautious about the situation and are probably going to overthink everything they do. You mentioned that you wanted to put in puzzles that they have to solve before running out of oxygen—what are those puzzles going to look like, and how do you plan to measure that time limit? If it's in-game rounds, that seems weird, since then you have to adjudicate how many rounds it takes the characters (who may be way the hell smarter than anyone sitting at the table, based on their INT/WIS/skills) to think something through and to perform whatever in-game actions are needed to proceed, which may be easier said than done. (The in-game actions might be the key, since if the puzzle revolves around manipulating physical objects like levers or blocks, you can base the timer around how many manipulations the players try, but that doesn't solve all the problems.) If it's a real-time timer, well, I've already said why I think that's generally a bad idea unless your group is drastically different from mine.

As has been mentioned, you'll also want to make sure that your group as a whole enjoys puzzles. Puzzles are difficult to do well—not impossible by any stretch of the imagination, and a puzzle that really clicks with the group is going to be an incredibly memorable part of a campaign. (By the same token, though, a puzzle that doesn't click with the group can end up being a huge pain point for the players—some of my friends, including me, still occasionally gripe about one really obnoxious and just plain not fun puzzle a certain GM inflicted on us something like six or so years ago.) Speaking from personal experience, not all puzzles are fun for all players, and if you've got players who already don't really relish puzzles (either they get anxious and overthink things, or they get bored and kind of disengage while the other players solve things, or they just get frustrated if they can't immediately get in the same brain space as the GM, or whatever), doing those puzzles underwater is likely to just exacerbate things. If you're already anticipating that one player is likely to be way better at (certain kinds of) puzzles than the other players, you'll want to make sure that the other players are going to enjoy letting the whiz kid handle things, and you'll want to make sure that things aren't going to become hopeless if the whiz kid just doesn't catch the clues you're expecting them to catch, or whatever.

Unless you're really confident that your group is going to like the kinds of puzzles that you're planning, and you're also really confident that the group will react well to a time limit, I would focus more on challenges than on puzzles, kind of like HammerdWharf mentioned. Enemies, environmental hazards, skill challenges, that sort of thing. You might throw in a puzzle or two, but if you do, you might want to make sure that the puzzle is optional (meaning that there's an alternate way past it without engaging the puzzle directly, should the players decide that they don't want to play that specific game).

Regarding Water Breathing, you might take a different tack. Don't look at the primary challenge as being "you can't breathe," at least not if you're planning on making the area in question bigger than about one dungeon room. Look at the primary challenge as being "you're underwater." Now, ordinarily, we associate being underwater with being unable to breathe, but that kind of gets difficult if you want the characters to spend the better part of a whole session in this sort of situation. But breathing is only one part of the challenges someone who prefers dry land is going to experience underwater. Visibility is likely to be poor. Movement is going to be slow and awkward, though you might get some mileage out of the fact that it's easier for most folks (especially most level 4 folks) to move in three dimensions when swimming than when on the ground. Combat is going to be much more challenging. Things like fire (torches, lanterns, spells, etc.) and paper (maps, scrolls, etc.) aren't going to be as useful in the water as they are out of the water. Currents are something you don't generally have to deal with on dry land. And so on. You can make the water be a challenge even if the characters are able to breathe—and in fact, taking away the breathing problem gives you a lot more room to play with the other elements of the challenge. If I may, I'd like to quote from Frank and K (http://www.tgdmb.com/viewtopic.php?t=35813):



To a limited extent an area can become more dangerous by making traps more ubiquitous. We say a "limited" extent because there is a profound sense of diminishing returns when the chance of encountering a trap equals one. Our classic example is the Citadel of Fire, the castle that is the home of the Efreeti King. It's on fire. Every square is on fire. Every door is on fire. And if you go there, you'll be on fire. To an extent, that means that the kind of dangerous area that you might have seen in the Lizard Temple when you were 4th level is now every square on the battlemat. That's… bad. But it's not unconquerably bad. It doesn't take a whole lot of Fire Resistance to survive in that kind of environment, and you don't have to be amazingly high level to get your grubby mitts on that kind of fire resistance. The fact that every single doorknob and chair is on fire in the Citadel essentially just means "Only Adventurers with Fire Resistance can Adventure here" or even "You must be at least as tall as this sign to attack the Citadel."

And the effect would be pretty much the same if you just had to wade through a moat of Fire. There are literally dozens of rooms in the Citadel of Fire that are on fire without this increasing the difficulty of your assault in any way. And that's OK. In fact, people would be slightly offended if large amounts of the Citadel of Fire were not in fact on fire, which would be the logical way to do it if you were handing out XP or construction costs on a per flaming room basis. It adds to the immersion to have some relatively homogenous fantasy environments.

Practically speaking, this means that by the time you have put in enough of a single type of difficulty that the players will not plausibly be able to complete their quest without taking appropriate precautions, the CR of the location shouldn't rise any more by adding more of the same difficulty. And that goes for more than just places being on fire. If there are enough pressure plates linked to arrows that the PCs aren't going to get through alive without the Rogue taking 20 on her Search checks, throwing in some more arrow traps (or tripwires, or anything else that the Rogue can find and bypass by just taking the time to search thoroughly) doesn't make the area any more difficult. A cave at the bottom of the sea isn't any more difficult when it's completely full of water than when it's mostly full of water – you still need water breathing just to get there.

Same sort of thing here. I know that you're saying that you don't want Water Breathing to simply trivialize being underwater, but what I'm saying is that letting Water Breathing handle the first challenge frees you up to explore what it means to be stuck underwater beyond simply "air-breathers have trouble here."

Overall, I absolutely respect what you're trying to do, but I feel like it's going to take a lot of careful work (and maybe some luck) to make the encounter actually be enjoyable the way you've kind of described it. There's not going to be only one right way to do this, and your group may enjoy things that the groups I'm used to wouldn't enjoy. But definitely think long and hard about what effect you want each element of the challenge to have at the table, and definitely do a little bit of contingency planning if your players don't seem to have as much fun as you were expecting.

[Mikalo]

Thanks for the answer!
Well yes, the puzzles I had in mind revolves around moving objects and stuff like this, I wanted to put hazards too obviously.
I get what you say though, the time limit can get weird and this is the first time I try something like this but I'm kind of a gambler. I had in mind to put four rooms with different puzzles/hazards, in each room there's a geyser providing air but the players actually have to reach them before finishing the rounds of air so they have to think very well about what they are going to do, it's a way to not give them an infinite number of moves. I am kinda lucky that my players are really smart so I always calibrate puzzles like this, if it's hard to me it is going to be easy for them... easy for me and laughable for them. Also I want to add that this is going to be a very fast map, back to slay monsters and stuff after that, giving them pressure is hopefully going to make things fast.

We are playing this evening! I'll write if it went well or nah

*the time limit is measured in rounds with a dice, the number of rounds they can breath is equal to their constitution score (wich is pretty high for all of them) they can share a round or more of air if they get close. I'm going to make them roll for initiative obviously