I'm not sure that it'd matter to be honest. I can conceive of two ways of carrying out that reaction, one of which most people here will have seen.
The exploding hydrogen balloon is simply that reaction, and doesn't require dry hydrogen. As you may have noticed it's a highly exothermic reaction, and proceeds readily (explosively, you might say) even in a humid atmosphere. It has a huge equilibrium constant - ever seen water spontaneously split into its constituent gases? (Most would consider it not to be in equilibrium at all)
I don't see why the second method would ever, ever be used, but you could potentially make water from oxygen and hydrogen in a controlled fashion, perhaps at a metal surface. This would probably be more sensitive to moisture, but given the huge equilibrium constant the presence of some water isn't going to affect yield too drastically.
Perhaps if the OP provides some context to the question it'd help us answer.