Popular beach clubs and catamaran tours sell out weeks early.
While "closed-loop" Caribbean cruises (sailing from and returning to the same U.S. port) technically allow U.S. citizens to board with a birth certificate and government ID, it would be very difficult to fly home from a foreign country without a passport book if you face a medical emergency or miss the ship.
The USD is universally embraced across the Caribbean. Local vendors and taxi drivers rarely have change for $20 or $50 bills, so keep a heavy supply of $1 and $5 bills handy
Many islands ban chemical sunscreens to protect coral reefs.
Ships do not always adjust to local island time zones.
If possible, fly into the departure city at least one day before embarkation