If you have a good dive shop near, check into it. Often they will give a free pool session as a teaser to get dive certification. The first step is always learning the signals and the proper way to clear your mask and your snorkel and how not to damage your eardrums in a free dive. This freebie can be invaluable. Even a quick, cheap PADI certification, even not planning to scuba, will teach all the things you need to know. You are spending a fortune on the cruise, the investment may be worthwhile and you will feel much more comfortable in the water.
Carla