The above is the norm for a crewed charter. They are doing a tremendous amount of work prepping the boat pre charter plus waiting on you hand and foot with all cooking and cleaning done. If you tip 20% of the charter amount they each get 10%.
If on the other hand you are simply placing a skipper on board to drive the boat the same percentages are fine but based off his actual skipper fee. In that case you are doing all the work and he is not splitting the tip.