I cultivate my own herb. I abhor the criminality associated with drug pushing gangs who are also involved with mass illegal DVD and music copying. Incidentally, do you check the copyright of all your open water student's music and film collections?
Of course diving and mind altering substances don't mix - just as flying to 10,000 feet and diving don't mix. The key here is that marijuana, as does every other drug, has a half-life which means it is metabolised and eliminated from the body. Obviously, you wouldn't smoke a joint just before a dive, just as you wouldn't drink a quart of vodka before a dive.
Don't get too hung up on the legalities.