Pre-paid gratuities can be added to the cost of your cruise and are all paid for at the time of final payment. NOTE: Pre-paid gratuities can not be adjusted once on board ship. If you don't Pre-pay gratuities then they will be added to your on board account. The on board account gratuities can be adjusted. Both Pre-paid, and on board gratuities cover your cabin steward, waiter, assistant waiter, and Head waiter.
Gratuity for drinks is an automatic 15% added on to any drink order. It is separate.