As I said, I think they'd be stuck to the bottom of the sea; but to expatiate on the subject, even if they have a great natural strenght doubled by the power armour I don't think they would be able to swim around without some special equipment. Power armour restrict their movements, and if that isn't an issue on ground it becomes quite a problem in water. Just try to imagine them doing the breaststroke... Not easy, eh? And I'm sure they wouldn't be able to crawl, first because they can't float, and second because of movement restriction. And don't even mention dog paddling or doing the backstroke!I think the issue isn't "can they operate under water" but " can they swim"
So I think that the problem is about movements, not power.