Depending on the stove, you may need to change more than the jets, should install a sniffer in the galley with a cutoff solenoid, depending on the boat, this ranges from easy to a week long ($$$) project
MOst are using 10lb and larger, as anything smaller will have to be filled frequently if you actually use propane. Good news propane is more readily available than CNG
the locker you point to only uses 1 lb cannister, almost too small to be useful. For the money you would be better to hang a tank on the rail. And yes, the vent has to be at the lowest point in the locker. I think you are seeing the feed line.
tanks have to be stored with vents also...
other than availability, my cng is fine