Re: How long (reasonably) does a tank of CNG last?
I can't speak for anyone else, but when I brought my Hunter down from Baltimore to the Keys in 2011 we were cooking at least one meal a day and after four months the CNG tank was still half full when I took it for a re-fill in Jacksonville on the way back up to Georgia(yes, there is a station there -- TECO People's Gas). My tank is old and looks like it came with the boat ... about the size of 50 cu ft. SCUBA tank.
So ... if you go to the Bahamas (where I don't know of any re-fill sites, you'll probably have to refill in either Jacksonville or Miami ....
If you search the Internet you'll find a map that identifies CNG re-fill sites throughout the US ... just do your research because not all of them are public access ....
Hope this helped.