Your options include:

1. Trying to negotiate an up-front resolution of the bill, based upon the work you authorized;

2. Refusing to pay the bill, then suing to recover your boat. (If you do this, you should expect to ultimately pay for the repair you authorized, and it may be quite a while before the gears of the court system grind forward to the point where you get your boat back.)

3. Pay the balance they claim under protest, then sue to try to recover the overpayment for the unauthorized repair.

They may claim that you authorized the repair, or in the alternative that the authorization you gave them was sufficient to cover all of the work performed.