Carnival recommends $12.00 per person, per day. The guidelines allocate $6.10 to dining room services, $3.90 to cabin services and $2 per day for alternative services, which include kitchen, entertainment, guest services and other hotel staff members.
This $12.00 gratuity is automatically added to your shipboard account, but it can be adjusted at the guest services desk if you do not feel its fair.
Alcohol purchases automatically include a 15 percent gratuity. You are required to show your passenger ID card to make the purchase. You are then required to sign a bill receipt for each transaction. Each bill has a line to enter an additional gratuity, but this is not expected because 15% is already included. In fact, I feel like this extra gratuity line a sneaky way for servers to get some additional gratuity from the uninformed.
Tipping a couple dollars for room service at delivery is always appreciated by the service staff. You can also leave some additional cash with your dinner server and your room attendant.