Hi Graham, air cooled car engines are fairly common in riggers - where the fins can get direct, cold air, and also speeds (or expected speeds) are significantly higher.
I don't think you'd be able to get enough cold air across the fins - in the types of boat we run, but I don't know of anyone who's actually tried it.
Marine conversions of car engines are often pretty straightforward. For engines which have a marine equivalent, it's usually as simple as taking off the car type finned heat sink head (which more often than not sits on the head button itself, and replacing it with the marine water chamber / cooling ring. These often transfer from one engine to another, because many engines have the same head bolt patterns - especially those with four head bolts. It's also within the capabilities of a well equipped home workshop (large enough lathe, or a mill plus workpiece rotating fixture), to make one.
Be careful with your choice of bobbin type couplings. A lot of them have ali end pieces....
Ian