Cruise lines tell us that dogs smell the luggage, but I've never seen them check the carry-on. Taking it off the ship after you get back would be more risky than taking it on board. You might want to watch the movie Midnight Express before you take grass across international boundaries. If you decide to blaze up while on board, the best place would be the sports deck at night. Also, do it on the down wind side so no one smells it.

You might not get caught bringing it on the ship in Amsterdam. But you might get caught when you get off the ship on your last day.

To be honest I would'nt risk it. You may be fine etting it onboard the ship, but when you get to the port in the UK you may be the unlucky one that get's searched. People assume that it is only the security at the airports that have the tighter security and the more restrictions but the ports and larger train stations have also tightened up their belt. Also if you are thinking of doing it onboard the ship the think of the other passengers. You would be foolish if you are going to do it as if you get caught you could face a criminal record!!! bear it in mind !