Another Air cooled engine that was common in boats till recently is the Enfield, some of the R.N. Motor Whalers have them installed, most of them now in private hands. The modern boats from 1970's onwards are usually Listers, they run for ever with very minimal maintenance, and are still built by hand in the factory, this also makes them expensive compared to a production line engine. Recommend a pair of ear defenders in close proximity, boy are they noisy when you are going full tilt boogy, bags of torque too.
Happiness is a Trawler conversion in a warm part of the Globe!