This is interesting and no more far fetched than the article I read a
few weeks ago about an engine that runs on compressed air. Of
course,storing either one would be that biggest problem. Warehouses
have been using either propane or CNG to power "hi-lo"s for a number
of years, so its not too unreasonable to try it in boats.
I doubt that it'll happen in my life-time, tho.....

Happy boating,
Norm