You can rent the snorkel equipment, floats, and bicycles at the beach on Castaway, or you can reserve them ahead of time. Either way, you pay with your KTTW card/account. There is a package that includes all 3 and saves a little money. However, we also bring our own snorkel equipment. The cost of one rental just about covers the cost of the equipment.
There is no guarantee that they will have the size/type bike you want, even if you reserve it in advance. They usually have a good selection, but on one cruise they didn't have anything left, and we lost what we had paid for the bikes! Didn't like that one, but didn't fuss about it.
So...by bringing our own snorkel equipment, we can go over to the bike area and see whether they can outfit us before we plunk down the KTTW card.