When you run the numbers make sure you can live with 45% of the purchase price on the boat for resale. Used charter boats are a commodity and after commissions netting only 45% is a realistic number.
The make or break part of ownership can be selling unused owners time. Push for 3 weeks a year that can be sold with as few restrictions as possible.
One last point, if your not going to use the boat 3 to 4,weeks a year minimum don't even think about buying! Rent!
George