Tip like you would a waiter or waitress. 10% is OK, 20% is great, Mates on charter boats make a very small base salary. The small salary gives them the extra incentive to make sure you have a good time. On most boats the crews split the tips, the split percentage depends on the boat and crew. The mate gets the bigger percent. If the boat is Captained by an owner and no mate please still tip! He or she is doing twice the work and trust me its a lot of work. Also remember that the crews day begins about an hour or so before you show up to the boat and ends 1-2 hours after, not counting any time they need to fix broken gear at home in the evening.