Been on 3 cruises myself, all to the Caribbean. My advice would be: Don't take anything across the border. You can buy a Pound of bud in Jamaica or Mexico for next to nothing, hell, you can just go for a cab ride and get stoned. Everybody has it in these places!!! Sweet huh????? However...... I think you've already read Pulverizer's comment "How funny it would be for your parents to witness you getting busted on the border for smuggling a small amount of weed/hash. First off, there is no lenience for a "small bag" at the border. They detect nano-grams on your person and they go buck-wild. You will be forever banished from the USA for the rest of your life and you will ruin your parents vacation. Win/win if you ask me.
A couple of great TV shows you should watch are:
on youtube search "Locked up Abroad"
"The Border - UK version/Australia version and USA version."
There are several people in these countries that sell people dope so that they can get paid by the police. Mind you, the number that get caught are relatively few. Is that a gamble you are willing to take? Is a joint worth doing that to yourself or your family?
Finding pot from crew members is actually not that easy to do. They are very well screened and sometimes strip searched. When your on the cruise, watch the crew members get off at a different door, shoes, socks off, and they also don't usually leave around the same time the customers do. Don't want them seeing that. If you MUST (I don't recommend it)
If you want pot in Cuba, hit the cities, not the all inclusive places. They sometimes get strip searched just to make sure they aren't stealing something or not sharing their tips. I would take hash onto a cruise and stick it you know where, I wouldn't smoke it, I'd eat it. Most cruise lines also x-ray baggage now. watch the exits from the countries you are visiting. If there are dogs there, don't do anything while visiting!
Just drink booze on the trip. You can get it pretty much anywhere on the ship if you even look 15. Wait for the on deck dances and shows. the Jamaicans are always pouring rum and others keep getting up to dance with fresh drinks on the table. do the math on that one. If that doesn't work, find some idiot college students that want to see a teenager drunk, they'll feed it to you. In Jamaica, go into a shop without your parents and look at t-shirts, when they come up (and they always will and will do anything for $) tell them you want a mickie of rum too. (make sure it is a plastic bottle with a plastic cap to get through the metal detector and shove it down your waist band before getting back on the ship. I learned that one from my Sunday school teaching mother who has been on about 20 cruises, that trick goes for everybody that wants to sneak booze on the ship. You are not allowed to take it back to your room. You have to turn it in when boarding and you get it back when you leave) At least this way, you may get a tongue lashing for making a "youthful mistake" and you won't jeopardize yourself or your family with international laws. If getting high is more important to you than risking someone other than yourself, you may want to reconsider your habit.