It depends on your needs and the crew's attentiveness and service provided. Most I have been on have a jar and the crew shares tips. I would do a minimum of 10% of the cruise price and more if you felt you had exceptional service. I have seen some tip nothing and others tip very generously, especially if a lot of alcohol is provided. If you happen to be on a smaller boat and the crew is the owner(s), I would still follow the same as I stated above but hand it directly to them as they may not provide a "tip jar".