Complex sentence: Shallow water blackouts can happen in extremely shallow water, even even on dry land following hyperventilation and apnoea (see Choking game) but the effect becomes much more dangerous in the ascent stage of a deep free dive.
Split into simpler sentences: Shallow water blackouts can happen in extremely shallow water; greyouts 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.