Tips will vary for each cruise line, but sometimes they will already be charged to you prior to the cruise. You can plan for around $12 to $15 per passenger/day. However if you are extremely happy with the service of a particular employee, feel free to add an additional tip.
The majority of cruise lines add a 15%-18% gratuity to beverages, spa packages and other services. Luxury and river cruise lines generally include tipping in the price.