Also a diver here. There are generally different biological mechanisms involved for “free diving blackout” vs “hypoxic blackout” (which is related to exertion w

Also a diver here. There are generally different biological mechanisms involved for “free diving blackout” vs “hypoxic blackout” (which is related to exertion while breathholding). Freedivers train to relax muscles and stay calm, so blackouts due to pressure changes are more common rather than to exertion. Swimming horizontally under water requires a lot of effort, which burns more oxygen. Combine high exertion with untrained individuals not aware of the risk, or misuse of hyperventilation (which doesn’t increase oxygen, but decreases CO2), and suppressing urge to breath, and you can also blackout. Here are a few good articles: http://www.freedive.net/chapters/SWB3.html http://www.freedive.net/chapters/SWB3.html https://www.redcross.org/take-a-class/resources/articles/shallow-water-hypoxic-blackout https://www.redcross.org/take-a-class/resources/articles/sha... It can even happen to more experienced individuals: https://www.wtkr.com/2015/11/24/navy-investigation-seals-who-died-in-virginia-beach-training-pool-were-likely-holding-their-breath https://www.wtkr.com/2015/11/24/navy-investigation-seals-who...