Bear in mind that power armour has servo motors in it, so the water resistance can be countered to a great extent like that.
By virtue of being void-proof they are water proof (hell, loot at how NASA test space suits and practices weightlessness).
Any weapon that works in space *should* work in water, lasers and energy weapons could be interesting though. Plasma should just evaporate itself a super-heated path through, and flamers use chemicals so don't require an external oxygen source (think thermite).
It would certainly be a different style of combat, but shouldn't be anything like impossible.