On our Honeymoon cruise on the Star Princess (The Love Boat!) we took just under an ounce and had no problems.
Here's what you do (this advice was passed down to me by my child's Godmother, who has been smoking MJ since the 60's, and is the only human being I know who can a) smoke me and my father under the table, and
Can drive a car and get you where you're going while tripping on acid. I trust her implcitly. Oh and she's been cruising since the 80's, goes at least once a year).
On the first day, go down to the front desk and tip your room steward a nice amount ($50 or $100, whatever you were planning tipping him at the end). Do it AT the desk because then they get bonus points or something. When you meet your steward, kind of feel them out, let them know you won't be any trouble, and you kind of like to be left alone.
If you have a balcony you should be OK to smoke out there, other decks MIGHT smell it, but the ship is usually going too fast for the smell to go very many places. Smoking in the bathroom is your other choice, turn the fans on, blow the smoke into the chimney, you should be fine.
Customs from the U.S. to the boat is non-existent. I'm not sure they even bother checking luggage, maybe the do. Just bring it on board with you - my wife brought it in her bra, they don't search you. Of course, take the double bag precaution, etc.
If you have a Medical Marijuana Recommendation from the state where you're leaving from (i.e. California I guess is the only one) you're fine actually. You can tell them in advance, and they shouldn't have any problems.
Half the crew tokes up anyway. Shit your steward may be 420 friendly.
As for in Jamaica, I really don't have any experience to pass along, thought I've heard that they are tough on stupid white people who get busted. My guess is that once you dock, find a taxi and get him to take you somewhere cool for lunch, and on the way see if he knows where you can buy some.
I wouldn't try to bring it back on board though, you do pass through a security check before you reboard.