for a liquid based realm, varying the nature of the liquid into regions could give one rooms and even traps. a de-oxygenated area where even water-breathing fails would serve as a nice trap. temperature variations would create visible differences, not to mention being potentially dangerous if extreme. various liquid and semi-liquid monsters abound. cubes, jellies, slimes, molds, even floating kelp forests could give you encounters and divisions of area. in a kelp forest, you could have many creatures being symbiotic. acid elementals would be particularly nasty in a liquid realm. whirlpools, riptides, and other current based movement effects can all add spice. bubbles can also be used for 'rooms' or special areas. especially if the bubbles consist of different gases which have their various effects. a bubble of pure oxygen would eventually poison the party, but not before making them drunk, dizzy, and full of colossally poor judgement. a vacuum bubble would also be interesting, especially if it popped unexpectedly. go from extreme negative barometric pressure to crushing pressure of the deep in a single round. *squish*. parasites in the water, electricity effects nearby...
i can come up with more if you like.
This is all great. Please, do come up with more if you can. This concept is really starting to congeal for me, so I should be able to start building it, soon.
Please, make no mistake, though. Just because the dungeon is entirely non-solid, doesn't mean I intended for it to be completely liquid. Bubbles of gas and vacuum are certainly a must, as are areas filled with slush, silt, froth, steam, energy and stuff that's somewhere right in the middle between liquid and gas.
let's see....
colossal+ sized glass lobsters (completely transparent unless they have recently eaten) call it 90% concealment, due to odd refractiveness of transparent armor shell... of course if what they recently ate had treasure... (this one is for grimtooth.
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leeches... touch attack, can't be felt, can only be spotted, blood sucking / con drain...
phase krakens...
ghost ships full of ghost pirates...
wandering wizard npc encounter in a souped up apparatus of kwalish...
rouge animated/possessed apparatus of kwalish...
colossal+ angler fish... make that lamp tentacle on its' head be about 100' long and it fades into the gloom... heeeeerre party party party...
shark swarm... maybe right after they pluck off the leeches?
electric poisonous jelly fish swarm = 3d maze, can't see, can't touch.
dinosaurs... make one purple and singing and just watch the party go nuts trying to kill it. but seriously, there are some very nasty water going dinosaurs out there to choose from.
hmmm, one of those massive tortoise islands with a sunken city on its' back, or maybe a sunken ship... (if you really want to tweak them, make it a modern submarine or a futuristic starship instead - you'd be keeping up the grand old tradition of every previous version of d&d to boot!)
underwater version of will-o-wisps...
borrow from the bermuda triangle - methane hydrate forms in the deep ocean, or in your situation in areas of intense pressure, possibly even forming crystals. disturb them, or if they move to a low pressure area, and they form a liquid bubble within water that has a density much lower than water, great for sinking ships, or suddenly plunging the party hundreds of feet as the surrounding water loses all buoyancy and becomes unbreathable. not so relevant to you, but it is also lighter than air, so it continues upwards from the ocean surface and kills all fire sources (engines) as it rises. the ocean surface also looks really weird when the methane hydrates "boil" out of the water.
that should be another decent series of suggestions.... ^^