High tide is 5 ft higher than low tide. Start at low tide. Use 1 ft of the 5 ft to go up to average water level. You still have 4 ft more to go to high tide. That means high tide is 4 ft above average water level. Then, the average water level is 4 ft below high tide. A height below another height is is a negative number of feet from that height. Since the average water height is 4 ft BELOW high tide, then relative to high tide, the average water level is -4 ft.
