Shallow-water blackout is loss of consciousness at a shallow depth due to hypoxia during a dive, which could be the result of any one of significantly differing causative circumstances.
The mechanism for this type of shallow water blackout is lack of arterial oxygen expedited by low carbon dioxide levels, as a consequence of voluntary hyperventilation before the dive.
Blackouts which occur in swimming pools are probably driven only by excessive hyperventilation, with no significant influence from pressure change.
[1][2][6][5] [7][8] One of the hazards of rebreather diving is a hypoxic loss of consciousness while ascending because of a sudden uncompensated drop of oxygen partial pressure in the breathing loop.
A similar effect can occur in open circuit scuba and surface-supplied dicing if a diver continues to breathe a hypoxic gas intended for avoiding oxygen toxicity in the deep sector, at a depth shallower than the minimum operating depth for the gas, but this is usually just called hypoxia.