This is up to you. The industry standard is 15-20% percent of the charter fee in the Caribbean and 10-15% in the Mediterranean. Ultimately, this is your call whether you give more or less. It is based on how you feel about the service—the crew aboard your yacht to serve you.
While cash is undoubtedly preferred, many crew members accept Paypal, Venmo, wire transfer, or credit cards. Alternatively, you can arrange pre-payment in part of full to your broker before your trip.