Niche market, but the extension hurts (even if going by marine registry). myyacht.us would be a good name for a self-indulgent yacht owner to showcase his big toy. If someone can afford a yacht, they can certainly pay a few bucks for a way to show it off. Some owner may be willing to pay a bundle if his yacht is named "My Yacht" You might check vessel registries to see if so. But that variable (and the extension) makes it hard to value. I'd expect under $200 but there's an outside chance it may be higher if you dangle the name in front of a owner whose yacht bears the name.
dreamyacht.us partly follows the same reasoning as myyacht, but might work better for a dealer. same price.