Seems like a shame to waste such a pun, but the wight template in Savage Species is humanoids only. Of course, the DM might still have ideas, customising it.
I know I'm late to the party and all, but as far as potions underwater goes, I'd be inclined to switch liquid potions for pills/gelcaps.
Then again, come to think of it, it's kind of odd that there are no arcane pills floating around D&D...
Or at least Alchemical powders...not that those would be effective underwater...I'm just saying.
Given what is known about cone snails and the various medusans, I could see intelligent creatures creating barbed harpoons that have poison bladders attached to them, so that when there is a successful strike, the poison automatically injects itself.
I'd expect "biotech" to be more common.
I'd expect underwater denizens to use more 3 dimensional fighting tactics.