Best time to buy a yacht is when you find one you want and the person is desperate to sell it
I can't speak to experience, but I'm not sure it matters as much. If you are in a seasonal area, you have to pay to store it over the winter and pay to keep it in the water
in the summer. If someone is selling, best time to buy is probably before they have to make that payment - spring would seem better as summer rates seem higher than storing for the winter.