If it’s a fully crewed charter where you have a Captain and cook onboard that literally do everything for you including cleaning your room the tip is based on the cost of the charter with 15% considered a minimum and 20% probably average. That’s a big tip however it’s split between the CA and cook and is part of their income.
A bareboat charter with a skipper added to just drive the boat normally has the tip calculated based on the cost to hire the skipper. The same tip amount would apply however if you have a outstanding skipper who goes out of the way to make it a great trip I would be tipping well north of 20% of his fee. The cost of the boat is irrelevant in this case.