Speculation about a buyer's motives is probably worse than free speculative legal
advice. Repairing the deal has to be a first attempt effort since in that case things could be worked out. Having a local agent that can verify the boat problem and perhaps oversee any repairs
might help as well as a legal
document that can be used to for certain close the deal and let you off the hook. So a little bit of legal work and diplomacy could sort this out and that is an attempt worth taking. Preparation for getting the boat back is also a possibility and that will require another set of paperwork else be accused of stealing a boat that never was sold. Might work out in the end but the middle could be a problem without limitation.
Should the diplomacy fail, the process of recovery needs to be at hand. With different countries and the distance issue you need the leg work done yesterday.
Negotiation works well when you understand the processes and costs of the alternatives. At some point in time there will be an obvious lessor of many evils that will emerge. Best to jump on it when it comes because it can always be worse.
To rephrase Dockhead "Free legal advice is worth what you pay for it and legal advice from the Internet
is worth even less." Talking about it here for free may help you feel better but you need a pro to help you navigate.