I have definitely seen common snapping turtles in areas of the Delaware and Chesapeake bays, however I have no idea what the salinity was in that particular section, as it can vary greatly, but regardless, I see absolutely no benefit in keeping them in brackish water for the simple fact that I don't know how long they can tolerate it and I doubt they have the salt-excreting glands that you would find in marine reptile such as your sea snakes, sea turtles, marine iguanas, saltwater crocs, etc. IMO, as far as Alligator snappers, I would say most definitely "no" simply for the sake of losing an expensive and endangered species, the cost is too great when weighed against the "risk and reward" factor.
I wouldn't even attempt to compare it to certain species of fish that can traverse "both realms" like Tarpon or Snook or Jacks, or on the other end, Mollies and Texas cichlids....the only thing they really have in common w/ reptiles is that they are both ectotherms. Everything else is entirely different genetically. Saltwater almost always causes dehydration in species that are not genetically built for it or in the case of the tigers of the Sunderban, where there is little to no fresh water, they drink the saltwater which arguably causes them to slowly go insane and become aggressive....these locales are responsible for more tiger attacks on humans than anywhere else currently (or so my source information states).
Probably one of those things "If you have to ask, it's probably best not to do it"
Cheers, mate
