The main foreign tourist seasons are from December to February, July to August, and Semana Santa (the week before Easter). Expect higher prices and more crowded beaches during these times. Also, note that most water sports and activities are prohibited throughout Semana Santa. The June-to-September hurricane season might be worth missing; though the chances of one blowing through are miniscule. It is good to remember though, that one little hurricane can wreck your whole holiday.
All in all, February and November are perhaps the best months to visit - both have great weather, thinner crowds and allow you to partake in either pro baseball, Carnival or whale-watching.