Depth Charge

[Palanan]

Are there any spells which could work as a depth charge, i.e., something which could be dropped onto a submerged target?

I'm open to all official 3.5 and Pathfinder sources. I haven't been able to find much myself, and I don't know how easy it would be to repurpose an existing spell.

[Saintheart]

There's the spell Gembomb, nice and low level, which could easily be repurposed as a depth charge by just saying it detonates underwater after travelling X distance.

Indeed by RAW you can throw it straight underwater without regard to physics. The Spell Compendium says "You can toss the bomb with perfect accuracy anywhere within 60 feet." Which would include throwing it at a three-dimensional spot below the surface of the water. Does [force] damage, too, so very much in keeping with compression blasts and not having to worry about [fire] being extinguished in water.

[Darrin]

Well, what comes to mind...

Delayed Blast Fireball (Core): You can set the glowing bead to detonate in five rounds. You can also pick it up and throw it, so... throw it into a body of water, and presumably you can work out with the DM it's buoyancy how long it takes to "sink". You could also summon a creature to "carry" the bead to your target. If it's close, an unseen servant might be able to carry it down to your target (no swim speed), but casting SMI for a celestial porpoise/fiendish octopus might work.

Explosive Runes (Core): A variation on above, summon a fiendish octopus, give him a note, tell him to go down to a certain depth and read the note. By RAW, a fiendish creature has Int 3, and thus can read and understand Common.

Fire Seeds (Core): The holly berry bomb version creates up to 4 seeds that could be bundled together and dropped into the water. When they get close enough to your target, say the command word. Damage would be 4d8 + (4xCL).

Depthsurge (Stormwrack): Kinda everything you want all in one package. Pick a point under the surface of the water within long range, everything within 20' takes 2d6 + CL, and any ship within that radius has to make a sinking check.

Attune Gem (Magic of Faerun): Essentially creates a spell-storing item, but you can set it to go off under certain conditions, much like a ward. Triggering condition can be a certain time, a creature within 5', or an object (if the spell targets an object). If you don't want to bother with taking the Attune Gem feat, you can do the same thing by casting the focal stone (http://archive.wizards.com/default.asp?x=dnd/wn/20040225a) spell.

Shallantha's Delicate Disk (Lost Empires of Faerun): Creates a ceramic disk that stores any spell up to 5th level. Break the disk, and the spell goes off. So give the disk to your fiendish octopus buddy, or maybe come up with some remote-detonation mechanism (fire seed, focal stone + shatter, etc.)

Skull Talisman (Frostsburn): Similar to creating a potion, but without any restrictions on what spell you want to store. The spell is triggered or released when something crushes or breaks the skull. So fiendish octopus volunteers again, or use a remote-detonation mechanism.

[Morty_Jhones]

its a bit of an odd use but

Goodbeery might be woth loking at as an anti undead, well mine would be more likely, but if your GM alows casting it on nuts as well then this could help screen a char from Zombie fish.

Flaming Sphere, would also be ok as long as your target is shalow compared to you, drops 30ft per round and guildable as it sinks.

Any creation spell + light spell, to make Flares to spot your traget easyer

Also

Ice storm, any traget at the serface of so only half Damage from all that Ice Pummeling them from below, but you dident say the depth charge had to go down :P

[farothel]

Any sonic spell that you can cast at a distance, as soundwaves underwater can blast quite hard (as diver, I've experienced how good sound travels underwater many times). A sound spell should do more damage underwater because of the denser medium.