Great South Bay begins with a narrow channel through the shoals off Amityville on the mainland and widens out to provide much of the shore’s deepest water and prime sailing grounds. Mid-bay depths run from 7 to 10 feet MLW with some holes even deeper but with shallower spots as well. The Great South Bay is 25 nm long and 5 nm at its widest point near the western end, the largest of the Long Island South Shore bays.