[nanidin]

I remember when going through scuba diving certification, the instructor mentioned that smokers generally get more time out of a tank because their lungs absorb oxygen more efficiently or something like that. And she smoked a lot.

[nradov]

There is no advantage to smoking as a diver. Gas consumption rate in scuba diving has nothing to do with lung absorption capacity (unless you have some kind of serious medical condition). We're usually not exerting at anywhere near VO2 Max. The urge to breathe faster is driven mainly by stress and CO2 levels. New divers are often nervous and flailing around due to having poor trim and buoyancy control. Experienced instructors don't have that problem regardless of smoking status.