people can carry 2H weapons in 1 hand too and even swing them around although probably not effectively. and if swimming in Heavy armor is in, which you acknowledge is possibly unlikely, why not dual 2H weild? its far easier and less believable than swimming in heavy armor imho.
Alright, if we're going to allow dual 2h weapons, then it must be done believable. In other words, the weapon will, at the very best, be too awkward to hold effectively, causing wide and erratic movements with a great loss of damage. to those who can't even hold it up, they'll just drab it around and flail around like a magikarp.
And the swimming in heavy armor, do you really want to get naked every time you go out to swim? That's more of annoyance than anything. But I have it even better, if you are carrying too much, you'll sink. Try carrying 100lb of stuff on your back, while trying to swim.