CNG is not practical in a boat:
Tanks are huge, heavy, expensive, and have low capacity. I don't like the idea of having 3600psi if there was a leak.

Propane may be better at 300 PSI with a high quality venting system as if it were in the trunk of a car but venting outside of the hull. Tanks are cheaper and hold more since the 300psi keeps the propane as a liquid.

LPG only makes sense IF....you had a cheap reliable (octane changes with benzine content) source.

with 108-110 octane you could safely increase the compression without getting detonation. The fuel has less BTU but if you were comparing LPG delivery to a carb in a BBC you would find little to no loss in efficiency....precise mixing of a propane gas with air is eaisier than mixing a liquid gasoline with air. Many skip the lpg injection and just use a mixer and a good intake manifold that fuels the cyl evenly. performance is very similar to EFI and mixers are cheap.

Propane loves boost. an intercooled procharger blow thru setup would be killer