You can not tip in the traditional manner on NCL. We just returned from the Sun and if you give cash to the waitstaff, room steward etc. they are supposed to put this money back into the pool to be divided. So what you have to do is go the front desk and fill out a form stating who you wish to tip, how much, if you want any tips removed etc. I also wrote down how much I disliked the policy.
Also when you pay for most bar dirnks and sometimes in the specialty restaurants there was a space to write in an additional tip. This they get to keep, if I understood the one bartender correctly.