One thing holding boats back from the kind of appreciation you see cars getting at Pebble Beach. The cars that are fetching gross sums of money are all restored to how they rolled out of the factory all those years ago. Boats tend to go through refits on a fairly regular basis.
Also, with a car you can moth ball and store in your garage for relatively low cost where as a boat it will cost you say 5-10% of purchase price per year to own even if you never sail (dock fees, insurance, dock lines, fenders, cleaning, waxing, bottom clean and paint). And there are a lot more car drivers than sailors in the world- bigger demand drives up price of historic cars.