Yes, you are generally free to get on and off the cruise ship as many times as you like while the vessel is docked or anchored at a port of call. Once the ship has been "cleared" by local authorities—a process that usually takes 30 to 60 minutes after arrival—the gangway is opened for passengers. You can leave in the morning for an excursion, return to the ship for lunch to save money, and then head back out into the city for afternoon shopping or sightseeing. The only absolute requirement is that you carry your ship ID card (often called a SeaPass or Sail & Sign card) and usually a government-issued photo ID. Every time you exit or re-enter, security will scan your ship card to keep a real-time log of who is on board. It is crucial to pay close attention to the "All Aboard" time, which is typically 30 to 60 minutes before the ship’s scheduled departure. Unlike a train or a bus, a cruise ship will rarely wait for late passengers, and if you miss the departure, it becomes your financial responsibility to travel to the next port to rejoin the cruise.