A workout on the sun deck, a conga line in the dining hall, a photo shoot with the captain, or a beauty contest for all ages: fun around-the-clock is guaranteed on a cruise, while you float along with your hotel room. The sea-faring holiday fortresses have come into vogue and business is booming. In the towering ship's wake, we are left behind with a hoard of digital memories and a cloud of exhaust fumes on the horizon.