The best answer is the difference in water depth. Generally, shallow water at one end gradually transitions to deep water at the other end. Some good swimming pools use railings to separate the deep water area from the shallow water area, but the shallow water area and the deep water area themselves gradually transition from shallow to deep. For example, shallow water generally transits from one end of the pool to the other end 1.7 meters. The same is true for deep water areas, except that the water is deeper and can meet different water levels.