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I am assuming that you signed a valid contract, that the bank fully complied with all the provisions, and that the amount they are asking for is accurate.
One thing I would want to look into is whether the sale, which occurred after you surrendered the yacht, was fair to you, and as specified in the contract. Was it a public or private sale? Was reasonable notice given so as to get the best possible price?
As far as options are concerned, there are several: