I just moved down here with my 60 gallon cube. Almost all of the stores down here have this service, in fact I bought 20 gallons of RO from them in preparation for the move. The salt water at Allfish is premixed IO. The other stores (for the same price at .69/gallon, including RO!) use ocean water that is filtered with UV (some use ozone) as well. Apparently there is a guy that goes out and collects in the ocean and sells to the stores. I just don't trust it yet.
On a side note, I wasn't happy at all with the livestock selection at Allfish. Still looking for the right store to compete with the Atlanta market, but Barrier Reef in Boca Raton is close. Might have to head further north up to Palm Beach...