Bring some cash!
In the hotels, restaurants, and shops, credit cards are accepted. Carry some cash for the markets, tours, public transportation, and getting into the beaches. You can get by with US dollars, but Jamaican dollars is widely used.
There are ATMS and currency exchanges in Montego Bay, Kingston and Ocho Rios. Airport rates are high so it's best to save the currency exchange when you're in the cities.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.