In the old days, cruise lines would expect passengers to slip tips into envelopes to reward crew members at the end of a cruise. That no longer works, given that it's likely you'll be served by many different staff members during your sailing. So tipping is not added automatically to your bill on most ships.

It's a mistake, though, to think of gratuities as an optional "extra," when in reality it's a service fee that makes up a good part of the hardworking crew's compensation. So be sure to factor in the cost of gratuities when planning your cruise!