In doing a bit of web search, the NG is compressed, not liquified, so it is possible to fill from a home or business line with the proper equipment; but the process takes many hours. Compressed Natural Gas (CNG) can be set up to fuel quickly, but it takes special equipment. The cost to run on NG is also about 1/3 less than gasoline.
In addition to a shorter range for the equivalent size gasoline tank, another disadvantage would be not running out of fuel. How does one find an emergency supply on the water? And,once the engine is converted to run on NG, it won't take regular gasoline.