Usually 15% is added to drinks as you buy and a set amount is charged to your account e.g. 13 dollars a day per person. We tipped extra to our waiter and housekeeper for exceptional service on the QM2 but it's not necessary. The amount is shared between everyone from cleaners to chefs.
It very much depends on which cruise line you are going on....Thomson and some others include tips in the fare, and sometimes offer free all inclusive where you don't pay anything at all for the drinks!!
On some ships there is a recommended amount for tips but is not automatically added to your bill. Ships which accept sterling or euros usually suggest lower amounts than American ships. Remember apparently British ships like P&O and Cunard are American in all but name as they are owned by Carnival.