Going back to Palma Vecchio's Sea Storm, it has just come to my attention that that canvas is believed by at least a quartet of scholars, including Bernard Berenson, to have been originally conceived by Giorgione. One other new wrinkle is the belief that the sea monster on the lower left hand side was added in 1833 by Sebastiano Santi. I have another book that seems to think it was acquired by the Accademia in 1829, but it was apparently then in the Albergo della Scuola Grande di San Marco, which took liberties with its restorations. This all still predates Turner's sea monster but it's unclear to me whether Santi's monster, and, by extension, Turner's, was following a fashion or whether Santi's monster was a unique image. This is a developing situation and you are advised to check back for updates.