Deep water blackout:

The mechanism for deep water blackout differs from that for shallow water blackouts and does not necessarily follow hyperventilation. However, hyperventilation will exacerbate it and the two should be considered together. Shallow water blackouts can happen in extremely shallow water; brownouts can be induced even on dry land following hyperventilation and apnoea. However, the effect becomes much more dangerous in the ascent stage of a deep free dive. Refer to deep water blackout for more detail. There is considerable confusion surrounding the terms shallow and deep water blackout and they are made to refer to different things, or used interchangeably, in different water sports circles. For the purposes of this article the two are separate phenomena with the following characteristics:

Deep water blackout occurs as the surface is approached following a breathe-hold dive of over ten metres and typically involves deep, free-divers practicing dynamic apnoea depth diving usually at sea. The immediate cause of deep water blackout is the rapid drop in the partial pressure of oxygen in the lungs on ascent.

Shallow water blackout only occurs where all phases of the dive have taken place in shallow water where depressurisation is not a factor and typically involves dynamic apnoea distance swimmers, usually in a swimming pool. The primary mechanism for shallow water blackout is hypocapnia brought about by hyperventilation prior to the dive.