For the price
of that long a rental, I'd also think it might pay to put down a down payment and buy a boat under a charter agreement, leave it in charter after your six months are up.
Or, you might ask about boats that are being DIScharged from charter plans, back to their owners. The owners might be interested in keeping the boat "in service" for another six months, and free to negotiate a deal on that.
Call & email
charter companies directly, that's cheap
enough these days. Invite their input for anything that's not standard, they may come up with other ideas like a lease/buy-back program.