When 2,094-passenger Arcadia launched in 2005, P&O Cruises embarked on a new era: its first adults-only ship. Targeting the younger end of the baby-boomer market, the very British ship offers traditions like black tie formal nights and assigned seating at dinner, along with a set of sophisticated options, including a restaurant helmed by a Michelin-starred chef, an art collection with more than 3,000 works, and a lavish spa that's central to the experience.