All this debate sounds like a plug for bio-diesel or cooking oil. Already widely used on the roads, either legal and taxed (biodiesel) or illegal and untaxed (cooking oil from the cash & carry), both are perfectly legal for marine use.

What about Tractor Vapourising Oil for petrol engines? Abolished officially about 40 years ago, lots of old tractor enthusiasts make up their own brews based on lamp parafin and toluene, I think. Tax free and legal if not used on the road. Petrol engines were converted to run on it by adding a thicker head gasket and a water-heated inlet manifold.