Just be sure to buy the insurance coverage and take a copy of your passport and your drivers license with you in the vehicle in case you are stopprd by state or federal policias. If you are given an FM1 visa, take that with you as well. Usually cruise ship passengers don't get FM1 visas. Also go over the car carefully prior to leaving the rental kiosk to avoid any additional charges for damage.
Consider hiring a taxi driver for the day. Often it is a bargain with fewer challenges.