mislav:Are we sure this guy was just hanging on the line? Perhaps he was practicing buoyancy, mask clearing etc.? I'm only guessing here...
Don't you "pratice" buoyancy control throughout every phase of every dive? Isn't it common enough to gave to clear a mask (or easy enough to flood it on purpose) anyplace?
If he needed to skip a dive to be at 20 ft to practice either he should be starting his count over from ZERO.