The atmosphere is cordial, relaxed. And you'll have all kinds of things in common to talk about. Around the pool. Or along the promenade rail. And don't be surprised if you find yourself making arrangements to meet them aboard ship again next year.
Passengers can vary from ship to ship and cruise to cruise.
Pack like you would for any resort. Cruise vacations are casual by day, whether you're on the ship or ashore. In the evening, ships vary as to dress. As on shore, attire is dictated by occasion. On some cruises, formal dinners or parties are part of the fun.