People have been busted and ended up with major problems by cruising into places with strict drug laws.
Bermuda, for example, once went onboard a ship and arrested a guy who had been identified as a "dealer" during the days of cruising to get to the island. The kept him in prison and it eventually cost him a lot of money to get out.
Cruise ships have a policy that if you are caught with drugs they can drop you off in the next port of call by handing you over to the local authorities - and you have no control over where that might be. Many places are not that lenient.