Here is some guidance from Voyage Yacht Charters when you have a basic captain on the boat with you.

What is the gratuity for a Captain and/or Chef onboard a Bareboat?

Tipping your crew is standard procedure in the yachting industry. Although gratuity/tips are welcomed they are not mandatory and are based upon the service that you, as the client feel you received.

For your convenience we recommend the following as a guideline:
Good Service 5 % – 7.5% (per crew member)
Excellent Service 7.5% – 12% (per crew member)
It is important to note that this percentage is based on your Charter Fee and is not based on the daily rate that you pay each crew member.

How do I pay gratuity for the crew?
VOYAGE charters prefer that you pay your hired crew gratuity in cash, however they are also happy to accept personal checks and or travellers checks. Regards paying by credit card and should you wish to make use of the VOYAGE credit card facility, please note an additional fee of 3.5% will be added.