Sailors that are free diving to clean their hulls, especially, might not have been trained and not understand this simple safety rule.
Humans have a thing called the Mammalian Dive Reflex which helps us free dive, among other things our bodies do as a result of being submerged in water is we stop breathing, naturally. Submerge a baby under water and they won’t try to breath, they will stop and hold their breath with out any training or instruction….. Unless you put a snorkel in their mouths.
The only thing stopping water from rushing into your mouth and therefore into your lungs is your tongue, this is an active block and if you have a shallow water blackout your tongue will relax and water will rush in potentially drowning you.
Shallow water blackout has a number of causes but the most common is hyperventilation. This causes a build up of oxygen in the lungs and therefore a lack of carbon dioxide. This happens rapidly and before you body has a chance to oxygenate you blood, so your blood oxygen level can drop too low on your next dive and you blackout, you don’t come around or try to breath because you have no carbon dioxide in your lungs which triggers the breathing response.
Free divers (should never dive alone) have survived shallow water blackouts by not being over weighted (so they float rather than sink), and once their bodies have stripped the excess oxygen out of the lungs , replacing with carbon dioxide the breathing response has woken them to the surface and driven them to breath again.
A good rule of thumb BTW, is twice as long breathing above the surface as your dive time, so you can balance out your blood Oxygen with your lungs before you dive again.