CCL is an american cruise line. They ask that you follow their customs that they have used when creating their line. The ticket cost is lower because your tips give better wages for the waiters if they deserve it.
My advice for THOSE NOT FROM AMERICA: HERE IS THE TRUTH IN OUR TIPPING!
It's easy to pick up on.
We have 3 standards of gratuity: 15%, 18% or 20%
Most restaurants will automatically add a 15% gratuity for parties of 8 or more.
You are expected to tip only when you think the waiter has done a good job. Tip more the further out of their way they had to go to accomodate you.
In my personal experience:
We always give the Maitre'D a gift because he REALLY goes out of his way for us each time.
Our waiter has had to put up with us for 4 cruises and does everything perfectly, quickly, and doing every special request we ask for. We tip him like most people would tip at the best restaurant for a party of 23 each night
Our steward was really sick during our cruise and his neighbor took over, and we tipped him as well. He had to go out of his way to take care of us. He deserved it.
We tip who we deserve, what they deserve. It's up to you. That tends to be the american custom of those who appreciate their service.