Shallow water blackout refers to a loss of consciousness due to dangerously low levels of oxygen in the bloodstream. This rarely occurs when scuba diving, but is a risk to skin divers who stay underwater too long without surfacing for a breath.

Shallow water blackout usually occurs as the skin diver nears the surface during ascent. As the ambient pressure decreases, the pressure of oxygen in the bloodstream also decreases. If the oxygen's pressure drops below the safe limit, the diver loses consciousness without any warning or signal to breathe. This results in drowning if the diver is not immediately removed from the water.