Actually a good idea to wear a t-shirt while snorkeling. You could burn and not even feel it then, because you are in the water.
I also have a suggestion. About a month before you cruise, start getting a base tan at a tanning spa...Just start with 5 minutes, no more and just go once a week until the week before you cruise, then twice. It always helps me NOT burn when down in the Caribbean. Just be careful and still use a good sun screen.