You made need to fight it. Generally, a mechanic's lien is a possessory lien enforced by the mechanic retaining possession of the vehicle until the work of the mechanic is paid. Since you have the boat, he would first have to sue you, get a judgment and then place a lien on the boat. If you are sued, then you may have to fight the matter. I hope this helps.
The answer to this question does not constitute legal advice nor does create a client relationship with the attorney answering this question. For more specific advice regarding your situation, please consult your lawyer.
I agree with my fellow attorney. He will have to sue you and when he does prepare to sue him with a counter claim. I would also through in a claim for him violating the Michigan Consumer Protection Act. This Act allows you to collect your attorney fees if you win that way you will be made whole in the end. I have used this Act successfully in the past.