
I believe the reason is that the lung diaphram gets too tired to pump
the liquid in and out and simply stops breathing after 2-3 minutes.
So if your in the vehicle ready to go they better not put you on 
hold, or else!! That's about it. Remember a liquid is several more times
as dense as a gas by its very nature. ~10 I think, depending on the gas
and liquid comparision of course!