As I cruised with MSC I saw they charge tips on your cruise card automatically if you don't expressly deny this permission.
The amount is per day, and depends on the age of the cruiser (standard for adults, reduced or zero for children).
The total amount is then distributed equally into the whole crew, to help those who work inside - and passengers neither realize they exist.
You can't tip directly the crew: I've been told from an officer that waiters etc. can be fired
if they're found getting money from cruisers.