U.S. citizens must have a valid U.S. passport at time of entry. There are no visa requirements if you have an onward or return ticket, confirmation of accommodation, and can produce evidence of your ability to maintain yourself.
NOTE: Be aware that Caribbean cruises that begin and end in the United States (closed loop cruises) do not require that you travel with a valid passport. However, should you need to disembark due to an emergency and you do not have a valid passport, you may encounter difficulties entering or remaining in a foreign country. You may also have difficulty attempting to re-enter the United States by air because many airlines will require a valid passport before allowing you to board the aircraft. As such, we strongly recommend that you always travel abroad with your valid passport.