We are currently on a Caribbean cruise operated out of Miami. On board there is a beer dispenser (PYOB - Pour Your Own Beer), which charges per ounce directly to my room. The machine is self-operated, although a staff member does have to refill the keg periodically.
There is no mention of a tip or gratuity, nor is there an option to amend or remove one, but the amount that is charged to my room is always 18% more than what is displayed on the machine.
In American Cruise ship culture, is this an appropriate practice?