Bulls are the only sharks that are happy in fresh water. Their young grow in estuaries. Bulls feed where fresh water enters into salt. So I would avoid swimming in the mouth of the Mullica, the Maurice and the Cohansey!
That's largely true, but not exclusively; some species of dogfish hae been found in fresh estuaries. Bulls are amazing creatures and have been found well over 2,000 miles upstream in the Amazon, and as far north as Cairo, Illinois up the Mississippi River.
There's some conjecture that a great white was responsible for some of the inland attacks at Matawan Creek NJ in 1916, but that has never been substantiated, and most icthyologists agree that a great white would have limited, or no tolerance or freshwater. It is noteworthy however, that juvenile porbeagles (a close relative of the great white) have been found in brackish estuaries.