It was caught in the canal in Cape Coral, Florida. The canals are clean brackish water — a mix of saltwater from the Gulf of Mexico and freshwater from the Caloosahatchee River — but the canal where the fish was caught has gulf access so it should be saltwater. The fish was about a foot long, a little less maybe.
It looks like the spotfin jawfish, Opistognathus robinsi. The larger ocellated spot on the foredorsal fin is a key identifier for this species. They are not so common in Florida, but do inhabit sandy bottoms, such as canals. Are you sure about the length? It looks about 5 inches from the photo provided, when compared with the the size of the hole on the dock. This species grows to about 5 to 6 inches.