A fixing group of a watercraft appliance for the attachment thereof to an outer side of a deck wall of a boat comprises a base of the watercraft appliance and at least one seat formed in the base and suitable for receiving a respective threaded fastening element, so that said element has a free end projecting from the base for insertion into a respective through hole formed in the deck wall, so as to be accessible from the inner side of such a deck wall for fixing the watercraft appliance to it. The at least one seat comprises an access at a peripheral surface of the base and a retaining portion suitable for cooperating through shape coupling with said threaded fastening element to prevent the rotation and axial sliding thereof with respect to the base.The invention also refers to a method for attaching a watercraft appliance comprising said fixing group to a deck wall of a boat by means of threaded fastening elements.