All this talk about alternate engines I'll throw in my quick 2 cents. I have a friend who ran a parasail boat with a diesel that was a ford powerstroke that some company "marinized". The boat was a complete heap of crap, everytime he took it out it broke down. This package was suppose to be a new "great affordable alternative" and it was cheap, but not affordable because it broke every day.
I don't think theres any doubt that you can convert any engine to "marine use" however marine engines (penta, mtu, cat, yanmar, merc, etc) were designed for a specific purpose. If you are looking for even a semi-reliable package, I would stick with the designs that have years of use and abuse under their belt already.