Having been clobbered on the head by a boat (admittedly on entry, not on ascent), I have no desire at all to repeat the experience. The one thing I do know is that my eyes are IN my head, and therefore if I'm looking up and see nothing, likely nothing is going to land on me. Although my buddies are my spare gas and my teammates, I can't guarantee that they will be able to spend as much time checking for a free path to the surface for my head that I can. And, as I said before, all the arguments for horizontal position on ascent begin to weaken substantially above ten feet. The only one that seems to hold is that it is probably faster to kick down than to sink, and I WOULD rather have the boat hit my feet than my head