$10.00 per day per person is not a lot to tip. Considering one can cough up a few grand for a trip, a few hundred more to tip those nice people who make a cruise special is not asking much.
15 percent is what many people tip on land for drinks, so it would be no different at sea. Yes, I wish the serving people were payed a living wage by their employers. But they are not, so the clients must make up for it.