There was a thread recently about this. There were a couple from the sounds of things that are doing just that but most would not reccomend doing that due to other contaniments in the water from runoff , pestacides , oils , ect.
I live in Connecticut and actually worked with a guy who used to get it from the ocean near where he lived and never had an issue even with the less than perfect water here in our state.
Also where exactly you are able to get it from also makes a big difference. Sure if you had a boat and able to get out to ( well lets just say the gulf stream ) open water you would be better that getting it from some little inlet from shore.
To me buying a salt mix and water filter is still fairly cheap in the long run and much more consistant.
Makes me think though " Oh honey I think we need to gat a boat so we can get water for the fish".