Disembarking from the crew position of one of a boat (rubber boat or armed assault craft)will sometimes put you above the boat, resulting in a falling animation and subsequent death by falling. The chances for that are about 1 time in 5.

I haven't played a lot of single player recently, so I'm not sure if the problem is happening there. However, we have this constantly happening in multiplayer, as I said 1 out of 5 times