Take a careful look at Carnivals Policy regarding drugs in your documents and on their web site. If caught you will likely be detained, put off the ship at the next port and turned over to the authorities at that port. If caught boarding the ship you will be detained and arrested at the post of embarkation. In ports like Jamaica, you are subject to search by Jamaican authorities when reboarding the ship. This subject is not "cute" and should not be taken lightly . Do you want to go to jail in some third world country?
In some of these third world countries you will be jailed until you are completely out of money. You must pay for a mattress. You must pay for meals. You must pay for toilet paper. You get the idea. If they know your family back home has money you may stay longer.
When you come back to the U.S. the dope sniffing puppies will be all over the terminal. If caught coming into the U.S. be prepared for a cleaner jail and smuggling charges.
P.S. Do not buy any drugs from any street people you might encounter in the islands either. They work for the police. They get a commission and their drugs back to sucker in the next U.S. tourist.