Like every time I try to go in the water with it I'm basically tying an anchor to myself so I can slowly drown.
Like most of the underwater segments in quests are designed with swimming in mind. So if I sink to the bottom while in Power Armor, there is no way I can get out.
The sea floor is also very steep and not fit for walking on. I've gotten stuck in a lot of sea floor crevices trying to get to some of the islands that you could easily swim to.