Back to the original question.
I feel it would be easier to survive swimming in water with a helm on, than with a breastplate. If I were wearing both, I'd try to get the breastplate off first. Even if all you can do is tread water, you should be able to keep your head above water, even with a helm. It is the breastplate that would ultimately drag you down.
Additionally, I'd get rid of both, but the breastplate first, then get my head above water. You can always remove the helm when you get your head above water.
Its interesting how many of us have had water survival course in one form or another.
When you spend months on board a ship, you ain't got much else to do except role-play, play penny poker (we weren't allowed to play for serious money), sit around and BS, etc. Thus, the reason I think many RPGers are military or ex-military.
rmfr