Dan,
Ron is correct that's where the CNG tank was originally stored that's where mine has been since I got the boat. A propane tank won't fit in that locker. CNG is more difficult to find but once you find a supplier that's a problem solved. The first place I'd look is at your marina boat fueling station, that's where I get mine. They swap the empties like propane. I also carry a spare tank which I can swap with the one in use if it runs out on a trip and also doubles capacity.
To be sure CNG is more expensive and harder to source but being lighter than air it's also safer than propane.