Travel by Air
All U.S. citizens are required to present a valid U.S. passport in order to enter or re-enter the United States when traveling by air. U.S. citizens do not need visas for short trips to The Bahamas for tourist/business purposes. It is important to note that although the Bahamian government only requires proof of citizenship and identity in order to enter The Bahamas, the U.S. government requires that Americans have a valid passport in order to fly home. Most airlines will not permit a U.S. citizen to fly to The Bahamas without a valid U.S. passport and risk getting stranded.
Travel by Sea
U.S. citizens may enter the Unites States from The Bahamas by sea using a passport or passport card. Travelers arriving via private watercraft are charged docking fees. All persons traveling by sea and staying overnight in the Bahamas must have a valid passport to travel.
The U.S. Passport Card may be used to enter the United States from Canada, Mexico, the Caribbean, and Bermuda at land border crossings or sea ports-of-entry and is more convenient and less expensive than a passport book. The passport card cannot be used for international travel by air.
If you are not a U.S. citizen it's a good idea to check with the Bahamas Consulate to confirm what documents you'll need to enter the Bahamas. You can get more information here.