I am affraid that the advice of the police is not strictly correct. Although by definition the offence of theft is the 'dishonest appropriation of property belonging to another', there have been circumstances where taking ones own property, which at that time is in third party hands, has been held to be theft. See R v Turner, which involves the Theft of Turners own car which he had left for repair at a garage. I would tread very carefully and if you find the vessel allow the police to recover it, dont do it yourself.