Crew members onboard a cruise ship work very hard for gratuities. In order to make providing gratuities onboard more convenient for passengers, most cruise lines will put the recommended amount for gratuities on your shipboard account, and then at the end of the cruise, the gratuities are paid along with the rest of your onboard purchases. A general rule of thumb is to plan for about $10 per person per day for gratuities. This amount provides gratuities for your stateroom steward, the dining room waiter, and the assistant waiter. Other shipboard personnel can be tipped for special services at your discretion.
If at any time you prefer to adjust the gratuities based on the level of service you received, you may visit the purser's desk (the hotel manager on the ship) and explain how you would like the gratuities adjusted, and they will happily accommodate your request.