The underwater level

[Trandir]

How would you deal with an underwater level in an usual nearly always on land campaign?
You got your usual land equipment, races, spells ecc... But one of the quests take you to an underwater location.
Just this one time you have to survive/fight/explore underwater and then you return to the usual abovewater adventuring.

[Efrate]

Depends on level. Necklace of adaptation even if on loan from your patron is fine, though you may need FoM to fight. Partially submerged dungeon with air pockets helps. Polymorphing into something with water breathing and a swim speed.

[Asmotherion]

-Freedom of Movement; The wording implies you don't need swim speed to move around and fight. if you don't have access to it (either in spell form or through the countless items that provide it) there's always the Swim spell.

-Water Breathing. Unless you don't need to breath (see below). Still useful at low levels as it can provide water breathing to allies.

-Alternatively Alter Self should be able to provide you with both a swim speed and the ability to breath underwater if you select a race that has them.

[Malphegor]

Polymorph us into some kind of aquatic creature. Doesn’t last long though but it protects our gear from water damage. As my current character is a tomb tainted dread necromancer, I think drowning isn’t an issue since my class negative energy stuff doesn’t need any components... I just need to be wary of unconsciousness

[pabelfly]

I like underwater breathing plants as a cheap underwater breathing solution, think they're in Stormrack

[Vizzerdrix]

How would I do it? First, I would commission a gnome submersible. Then I would shrink it down to about a yard or so long, maybe smaller. After that, PAO everyone twice into either Eneko or half giants, then use return to nature until we where of proper size to crew the ship.

I would add a few things. Spell turrets, an endless bottle of air, and an endless bottle of water, as well as some way to sturdily anchor the vessel against massive forces. I would employ the EBoW as a forward facing battering ram of sorts when anchored, or an emergency escape propellant when unanchored.

[TheCount]

As Efrate mentioned, a nacklance of adaption would be my first go to
There are also the spells mentioned above. there is also control water.
magical plants were mentioned as well.
whacky ideas (like the gnome submarine) is another way to go and were mentioned already. (or just get a disposable minion to plant a beacon stone for your teleporter)

But Elementals are a thing!
water or earth, those two would be my choice, the water if you prefer head on attacks or earth if you like stealth, use both for best effect (in case something did break into your newly made tunel).