here's a true freshwater story.. I'm sure this guy would've been a sinker in the salt.. I had a college football player/muscle builder wanting to give snorkeling a go.. he wasn't that tall, not a great swimmer but a fair swimmer.. I found a 3mil wetsuit that kinda fit him.. he sunk like a rock... I found a large life vest (one of those not so fancy ski vests, but still, a good life jacket) that was just barely big enough to go around his chest.. and even with 3mil wetsuit and life jacket, he sunk like a rock.. .unless he was moving, he was definetly sinking..and even while moving it was a struggle to keep on top the water.. unbelievable.
Myself, fresh water, 3mil suit.. let 1/2 my air out, I'm going down, especially if I'm verticle.