I would seriously advise getting a 30-day package at a local tanning salon before you go. Even in the summer, there's a quarter of the folks who get absolutely lobstered the first day or two, and then they are unable to be out in the sun the rest of the cruise.
Thirty-two days before your plane leaves, go to Hollywood Tan, get a package, and get a nice coat of dark on you before your cruise. Then with the SPF on deck, you'll avoid getting blistered.