There are some master courses specifically in yacht design. I saw three in Italy mainly concentrating on aesthetically aspects, and one in UK for more technical side. Naval Architecture is more general science and if You want to design yachts You might not need to study probabilistic stability or loads on offshore drilling rigs.
Meanwhile, no course only will make good professional in boat design. Boating experience is essential, construction experience, apprenticeship with right design team. If You have those plus formal training, then You can succeed.