I just stumbled across this thread. Much good analysis. It brings to mind a simple principle that, once we realized it, has made diving from day boats and liveaboards more enjoyable for my buddy and me: If we were ever to find ourselves facing real trouble, whether that is running so low on air that we could no longer make a normal ascent, becoming physically exhausted by an unexpected current, or whatever, our plan is to promptly ascend and signal the boat (SMB, Nautilus, etc.), rather than try to make our way back to the boat. What we realized was, of course, not simply that this is possible but rather that any potential embarrassment is far more palatable than physically hurting ourselves. If the surface current were so swift that we would quickly be carried out of sight of the boat, we probably wouldn't have dived in the first place. But I suppose if we were knowingly diving in such conditions, we would bring a huge SMB, Nautilus, PLB, DiveAlert, etc.--that's what they're for.