I personally wouldn't sweat it. Sounds like the captain would basically be on the boat for about three hours and then you would be free. You might even be able to get underway quicker if the briefing can be done by the captain, or if the briefing can be shortened because you'll have a captain on board.
However, if you really don't like the idea I would think that you'd have grounds to cancel the charter for a full refund. Just tell them that they changed the contract and you're not satisfied with the new one. In reality it seems that you could enforce the original contract since it is a legal and binding contract, but I think it would be wiser to just find a different boat than take that route.
