Are you actually doing a "private charter trip" or going just as a passenger(s) on a "group tour" day sail boat trip? If a group trip (say on a catamaran with 20+ pax), a tip of about $5US to $10US per person is given to the crew is adequate. If a private charter (say on a sailboat with 2 to 6 pax) a tip of 15% to 20% of the total charter price would be more appropriate - give a portion (say 1/3) to the Mate and the rest to the Captain.