Originally Posted by BigMac
Cruise ships traveling internationally cannot allow banned substances on board.
Law on the high-seas is tricky. I don't know of any port where marijuana could legally be boarded, save California. Similarly, I don't think any of the common registrant nations specifically prohibit it in maritime law. But, no cruise line permits it as a matter of regulation, regardless. Also, the United States claims special arrest rights if it deems you intend to import any federally-illegal drug into its territory (at direct opposition to California state law -- man up, states rights activists!), so some U.S. ports could be an issue if you don't smoke it up.
Of course, realistically, nobody will know.
If you manage to sneak it on and are in open waters, it is just a ship rule at that point, no different from saving pool chairs with a towel. On the open seas you are literally outside the arm of the law. And the captain may choose to enforce the rule as loosely or harshly as he sees fit.