Waves passing overhead cause surge. Surge is a reliable reference because the back-and-forth motion swings to and from shore or shallow areas. You feel it most strongly in shallow water. However, large swells can make surge noticeable in even relatively deep water. When diving in surge, note the water movement to determine the direction toward shore. If you’re disoriented to the point that you don’t know which “swing” is shoreward, follow the surge in one direction and watch your depth gauge. In