Most often the reason is a dead battery plus an error by the diver that causes their handheld "twist on/off" flashlight to flood. The diver twists the light in one direction to activate and nothing happens, so the diver twists the light in the other direction attempting to activate and nothing happens. Then the diver keeps twisting in wrong direction, breaks the O-ring seal and it floods. Other less common reasons are failure to maintain correct amount of lubrication on the sealing O-ring(s); dirt, hair, or salt introduced on the O-ring(s) or sealing surfaces when loading batteries; over-tightening the light head and body together; and finally leaving batteries (especially alkaline chemistry batteries) stored in the light over a long period of time.