In order to get into Cuba from a cruise ship you must purchase a Cuban Visa. (approx. $95)   If you don’t want to purchase it yourself,  most of the cruise lines will purchase it for you and give it to you when you check in for the cruise.  When you get off the cruise ship or back on the cruise ship you will have to pass through security and customs each time. The first time you get off they will TAKE your Visa, stamp your passport and take your picture.   Each subsequent time they will just check your passport to make sure it is stamped.