HAVANA — The first U.S. cruise ship in nearly 40 years crossed the Florida Straits from Miami and docked in Havana on Monday, restarting commercial travel on waters that served as a stage for a half-century of Cold War hostility. Hundreds of workers and passersby gathered to watch, some cheering, as the gleaming white 704-passenger ship operated by Carnival’s Fathom subsidiary pulled into the dock — the first step toward a future in which thousands of ships a year could cross the Florida Straits, long closed to most U.S.-Cuba traffic due to tensions that once brought the world to the brink of nuclear war.