Either boat can be taken for a morning, an afternoon or a full day, and then the route is yours. People use it for family days, for a photographer who wants a particular light, and for getting a group of twelve to an island the ferry reaches twice a week.
The price is the boat, the crew and the fuel for a set range. Going further is quoted rather than refused.
Ask at the office with your dates. A charter takes one boat off the public board, so the sooner the better.