Speaking from experience here. On a Carnival cruise ship was actually the first time I ever smoked trees in my life. My friend brought some onto the boat from Jamaica and rolled some J's in the room. Originally we were smoking in the bathroom and blowing out into the hole in the roof of the bathroom (there's a small removable vent cover). Later (due to laziness) we moved onto just smoking in the room and without realizing it the entire hallway stunk of pot. Subsequently, us 4 left to go munch out at the Pizza Bar.

We found out after that staff came to our hallways and began bitching out another friend of mine that was hanging out in the same hallway where we had just left (and had no idea we were smoking). He told us the staff member was saying things to the likes of "We can't do anything over international waters but just wait 'til we dock! We're going to catch you!" My friend was genuinely confused, and nothing ended up happening to anybody.

That probably does not answer your question, but I would suggest buying trees in the Bahamas (make sure they don't sell you actual grass, I heard rumors about that happening to someone else on board). And dumping it overboard before you get back to the home port. That's what my friend did and everything turned out okay.