A common occurrence during underwater training is shallow-water blackout (SWB), which is a loss of consciousness caused towards the end of a breath-hold dive in shallow water (usually less than 16 feet). The cause of this cerebral hypoxia, or a lack of oxygen to the brain. It is also called latent hypoxia.  Victims are often established practitioners of underwaters, are fit, strong swimmers, and have not experienced problems before. They usually do not experience an urgent need to breathe immediately prior to the blackout. A lot of drowning and near drowning incidents occur among people who black out underwater while free-diving or doing breath-hold pool laps. Blacking out near the end of a breath-hold dive is common, which is why it is important to have a lifeguard or buddy present AT ALL TIMES DURING UNDERWATER TRAINING.
Shallow water blackout is related to deep water blackout in its characteristics, mechanism and prevention; deep water blackout differs from shallow water blackout, in that it is precipitated by depressurization on ascent from depth. Blackout may also be referred to as a syncope or fainting.
Some blackouts have been directly associated with the practice of hyperventilation. Some who have experienced a shallow water blackout often report using hyperventilation as a method to increase the time they can spend underwater. Hyperventilation involves taking fast, deep breaths before executing the underwater or dive. The reason is the myth that hyperventilation will increase oxygen saturation in the bloodstream. In truth, a healthy human body is saturated with as much oxygen as is necessary while normal breathing occurs during any strenuous activity. With hyperventilation, these people are extending their dive by closing down the body's natural breathing mechanism, not by increasing oxygen load.