- Get a lawyer (at least so you say you have a lawyer)
- Start a log (of every action taken / incident)
- Capture as many names as addresses as possible (not everything stays on the
internet forever).
- Check your own insurances (including household) to see if you have any
legal disputes cover (possibly not so common in the US - but having $100k "free" for lawyers does concentrate minds

).
- Maybe even work out the likely
route (given his previous progress reports) and whilst he may be lying that the boat is somewhere secure (no claim for the cost of that!) can't be a squillion places that the boat is located.
- If you have any debt on the boat then contact your lender - they may well be very interested in someone taking "their" asset (and likely have experiance of folks doing just that!).
- Contact the insurance company for the load (and also for the boat).
- Formally contact the Transport Company in writing (you or lawyer) - especially to find out if they sub contracted the job to someone not directly employed by them. Even if the load was insured seperately, ask for the name and contact details of there insurance company.....the more people you can sue for loss of boat - or more likely any damage when recovered the better.
The good news is that he is relying on the "possession is 9/10ths of the law" angle - which though annoying is IMO nonetheless encouraging that he has doubts over the strength of his claim. Even if the trailor was overloaded I think a good argument that not your problem.....but don't get into specifics on that angle, just in case

. The other good news is that whilst the dispute runs on he is getting out of pocket (expenses and
lost work), and even if it turns out his case is valid

he will be having doubts, plus sooner or later cashflow problems (likely
solved by offloading the boat somewhere).
The trick will be getting the boat back (and undamaged!) - I would suggest that he deliver the boat as agreed and for which the 2nd payment
would be made.......and afterwards you and him would argue it out in court - but sweeten the deal by making clear that you have no problem paying if you have to (but also make clear you don't beleive you do!, and even if so it will be an insurance company thing for you). none of that has to be strictly true

......but got to bait the hook with something!....and "free" money from an insurance company as good as any.
In the interim speak to a lawyer and get your ducks (and paperwork) lined up in a row so if you do need to go
legal and hard ball you can do........I am sure loads held "hostage" is not exactly new legal territory! (and he will probably have a better working knowledge of the legal side than you - at the moment anyway).
Obviously I am not a lawyer! I am not even in the US!. Just that I have found that with disputes there is a mix of the practicality and the legal. Step 1 is to look serious by being organised, persistent and rational (no matter how angry you are) - as it says you are neither going away, are not a clueless idiot nor are willing to be pushed around.
Good luck - and let us all know how it goes.