We normally tip 100.00 a day and than if we have money left in our kitty we give that to the Captain at the end of the trip also.
The first time we chartered on our own and had to hire a Captain I read someplace to remember that they are working 24 / 7 while you are on vacation 24/7. You may think they are sleeping but even when they are sleeping they are still on watch.
I contacted our Captain ahead of time to see if he had any food allergies or wanted anything special, asked what he drank etc. I wanted it to be a good experience for him with us too!
He ate every meal with us. Breakfasts were on board and lunch or dinner was out. We took turns on who would add his meal to their tab.
We took care of him, he took care of us....found fresh aloe for our sunburn, climbed a tree and got us fresh coconuts, got us a discount on a slip, deal on a rental car....the list goes on.

If you are not going to include your captain in your meal out or on board one night my suggestion is to communicate that early in the day and ask them how they would like to handle the cost of their meal that day. They may appreciate you giving them the money upfront that day....and most would not ask. Times are hard now there and they may not be carrying a lot of cash with them.