Or, maybe more importantly, a seller who doesn't bother keeping the visible parts of his boat clean probably also didn't bother keeping the boat well maintained in general.clean is a must, simply because most buyers lack the vision to see what a cheap boat could look like with just a little bleach and a scrub brush.
Most buyers are going to buy the boat that they can see themselves using to achieve whatever sailing goals they have set. Those range of goals vary from day sailing to weekends to blue water cruising. The attributes of a boat that fit each one of those goals are probably even more different than the attributes that make houses attractive. As an example, a big airy cabin is a great feature to someone that wants to enjoy some time at anchor or at a dock for a weekend, but a terrible detriment to someone who's going to get beat up in heavy seas.
One goal that very few buyers will have is to really want to spend more time and money fixing up problems and recovering from "deferred maintenance". For that reason I agree with some of the others above that having the boat clean and well maintained/repaired is the one key attribute that's likely to get the most buyers engaged.