If you are doing an excursion, they usually provide the equipment for you. But che to make sure. Usually you can find the excursion's website. On the Conquest (and I am sure other Carnival ships too) you can buy a snorkel for $5 - brand new. Sometimes while in port, if you are snorkeling or diving not with an excursion, and do not want to lug your own stuff around, you can find a shop to rent from. I know you can rent in Grand Cayman.