Mike,
The Nelson marine motors made by Henry Nelson are not converted airplane motors, they were designed as boat motors from the start, both the inboard and the outboard, the cranks and the crankcase are of a different design made specifically for marine use.
The other Nelson in MN is working on a prototype Vision Tech 45, that motor was designed in a University lab (engineering class I think) and decided to put it in the market. There was a paper posted somewhere a couple of years ago about the work on the motor, and to complicate things, of course, the guys name is also Nelson.
Gene,
You're right about that those motors can fly, seeing as Henrys' airplane motors hold bunch of airplane world speed records, I can see how they can make a boat fly also, now if I can keep my Leecraft from thinking it's an airplane, I might get me a couple of thropies.
Bill