Neil, I don't know the specifics of bankruptcy law in France or the EU...but in the US you would be considered "an unsecured creditor". (You have a "claim" but no rights to items which you paid the bankrupt party.

If you own the boat, you presumably can prove it and reclaim your property -- but you may need to prove that, so be prepared.

It is possible that the "new yard owners" may do something for you on a discount or otherwise -- but that's a "I've go a problem and I need your help".