Unfortunately, the yard has a "mechanics lien" on the boat, which in effect securitizes their bill with the boat. It's my take that this lien stays with the boat no matter who may be the current
owner. Did you use a marine
title company to transfer title on this vessel? If so, they would have done a search of any outstanding liens. I would double-check with them if there was any lien filed prior to your ownership
Practically speaking, the amounts in dispute are small, so small claims court may be your best recourse. However, you can only sue the manufacturer to live up to their agreement to pay (as I'm assuming they didn't put a cap on it). If they are not a US entity you'll have a problem with that. Maybe, it was a U.S. arm of the company that authorized this work, then you'd have a party to sue for the deficit. You could sue the yard performing the work to take the lesser payment if you can show that they had agreed to a lower amount initially, but from your facts, it doesn't sound like it's the case. A small claims court will mostly just make monetary awards and not directives of equity (meaning an order of replevin to force them to give the boat to you). However, I'm not fully acquainted with FL law, so you'll need to check if a motion in equity is possible in small claims court there.
It may be best to just negotiate with the yard and the manufacturer as best you can with threats of lawsuits (beyond small claims) that no one really wants and will cost way more in legal fees
. Tell them your cousin, or son in law, or best friend is a lawyer and will make life difficult for them and it won't even cost you a penny. They may think twice about how you're being treated.