Yes, I agree with jazzgal. 20% is about right. Usually a tip jar is passed around, or at the exit when you leave the boat. I would put the tip in there. I would not tip an individual as it might cause problems if the Captain sees that, he might feel he should get a portion of it.
I'm sure the crew knows the guidelines of what is their expected portion of the tips, and they probably divvy it up according to those guidelines. You're on vacation, they're not-be generous !!!