§ 80. Three feet depth to be maintained. Whenever a municipal corporation shall divert or cause to be diverted the water, or a portion thereof, of a fresh water stream or streams outside the limits of an incorporated city flowing into a tidewater creek or estuary which tidewater creek or estuary before such diversion was navigable for vessels of twenty or more tons burden, it shall be the duty of the corporation so diverting or causing to be diverted such stream or streams of fresh water to keep said navigable tidewater creek or estuary deepened to the depth of at least three feet at low water mark from its mouth to the head of tidewater for the full natural width of said creek, and to maintain such navigable depth of water at all times, but in the county of Nassau it shall not be compulsory to maintain such depth to any greater width than fifty feet.