NCL uses a distinction between "tips" and their "service charge". Everyone 3 years old and above is assessed a service charge of $12 per day. You can't opt out of it. You can adjust the amount downward if you have an issue that can't be resolved. The reports I've read indicate adjustments can be hard to get. HERE
are the details. If you scroll up on the link you'll see their "tipping" policy.
Note that they expect you to immediately tell them of a problem and give them a chance to fix it. If you show up on the last night of the cruise and want to reduce the service charge because your waiter forgot your appetizer three night earlier and you are just now telling them, you will likely find a stone wall.