Most of the cruise lines do charge you a per person per day gratuity charge (from say $12 - $20 US per day). This covers your room stewards, dining room and other serving staff. The people sharing these tips work extremely hard at their jobs – often with very few days off.
Gratuities are counted on to achieve a living wage. Some cruise lines, while they will charge it directly to your onboard account, will allow you to adjust the gratuity total –evendown to zero. If you thought the service was sub-par, you have the option to adjust it, but I urge you not to – the money does go to the people who deserve it.