what action can I take against a mechanic's lien filed against my boat

The exact facts of this situation must be clearly and completely stated in order to correctly analyze the case under maritime law. Here, if the vessel can be considered a navigable vessel, and if a mechanic actually performed services on behalf of the vessel that were then not paid for, than the mechanic can generally assert a maritime lien on that vessel. Such a lien would arise as a matter of law, and would not be fraudulent. However, in order for a sale of such a vessel to take place, the mechanic/repairman must have first filed an action in rem against the vessel, arrested it, and then successfully motioned the court (Federal Court, due to admiralty jurisdiction) to order to effectuate a judicial sale. All of these procedures take time, and you would have been made aware of them at each stage. If you were not, than there is a possibility that the vessel was improperly seized by the repair-shop, and you might have an action against them for conversion. You should discuss the complete facts of the matter with a qualified, Florida-licensed, maritime attorney to determine your rights.

Adding on to the previous answer, there is such thing as "Wrongful Arrest" of a vessel. Admiralty/Maritime law is a very specialized area of practice, so I suggest that you contact an attorney that primarily handles maritime matters. Good luck.