U.S. citizens arriving by air and sea to Bermuda need a valid passport, though cruise-ship passengers on closed-loop cruises (those departing and arriving in the same U.S. port) need only have proof of citizenship and identity (a government-issued photo ID and a birth certificate with a raised seal). U.S. citizens do not need a visa to enter Bermuda for a period less than 90 days.
U.S. Passport Information
U.S. Department of State. 877/487–2778; travel.state.gov/passport.