The EU rule: If the sale of a second-hand boat occurs between two private individuals, VAT is assumed to have been paid at the origin, and it is sufficient to show the sales contract.
The fact: If you have not any proof of VAT payment (es: the original invoice, the receipt from the customs office, or any receipt of the payment), and (as someone said before) a statement in a
contract that VAT has been paid is not proof; well you could find yourself in troubles.
In some EU country, without a regular proof, customs officers could seize the boat, and it has happened before, rarely but it has happened
Don't ask me why some EU customs go against the
rule, but they did............