Well why not, here is my take on it. Keep in mind when I say "you" I mean anyone who thinks this might be fun, not just the OP. For all I know it is legal where he lives and this is a valid question.
I doubt that the captain would see his vessel siezed, unless there were hundreds of pounds of the stuff on board. Enough to get a few people lit for a week is not going to get the captain in trouble. However...the fact the stoner might do something stupid and get hurt or hurt someone else could cause other types of legal problems, so I bet the answer from the liveaboard will always be "no."
You had better inform your dive buddy as well. I am not interested in all the stoners saying how they can do whatever while on weed or that it only lasts for so long...meh I don't care what you think, we all know it destroys brain cells so you are bringing a knife to a gun fight. How you might be affected underwater after having a joint, either right away or hours before is of concern to everyone in your dive group. Just because you don't "feel stoned" doesn't mean that once you hit ~60 FSW your world won't go wonky. If you are willing to take that risk toke away, just let me know so I can stay the hell away from you.
I too have random drug tests for work, as well as a security clearance. There is a 0 tolerance for THC, any amount they can detect is enough cause for dismissal. How much second hand smoke do I have to inhale accidentally to show up on one of these tests? I do not know and the short answer is I do not care. If I am at a party and somebody produces an illegal substance I can simply leave. On a boat I do not have that option so I am going to invite you to dump your stash overboard. I will also report you so that if I do get tested on my return to work and pee "hot" I have a police report to clear me. I will not let your bad habits ruin my career.
I too am paying thousands of dollars for this vacation, and to have someone so cavalier about the laws behave in a way that it might cause undo stress or a serious interuption of that vacation will be dealt with swiftly and harshly. If we are discussing whether weed should legalized, or just de-criminalized, my answers may surprise you. However right this minute almost everywhere (and probably on just about every liveaboard destination) it is not legal. So if you think you understand all the risks and are willing to take them for a few hours of whatever it is weed gives you, then consider this your only warning if you and I find ourselves on vacation together...I will test your convictions about risk taking.
To sum up, I have far too much at stake to gamble my life, family and career so someone else can get an illegal high on vacation. I don't know how much is too much and I am not willing to believe the tokers have my best interests in mind when they make up the facts in a way to suit their argument. I won't stand by and risk anything, I'll just call the cops...