From breathing compressed oxygen or from descending?Originally Posted by Acrophobe
It doesn't matter because the answer to both is no. For one, as they descend they are going into increasing air pressure. Secondly, the compressed air they breath is not significant. It only lowers their effective altitude by a few thousand feet. They are still extracting air from the atmosphere. Divers get 100% of their air from the tanks.
Also, the differences in air pressure are nothing compared to the pressure under water. There's as much pressure under 15 feet of water as there is in the entire atmosphere.