I loved the waverunner excursion in Labadee, check into that, great fun and a nice way to see more of the country by water. Its pretty popular, so if you decide to do it, pre-book a time slot that you want.
The first tenders they allow cruisers with shore excursions off first. Sometimes they will give out tickets to get off, first come, first serve type. Usually once in port, give it an hour or so and you should be able to just walk off onto a tender. If you want, you can go back and forth, but easier just to get off for the day and return to the ship when your finished in port.