The maximum depth a person can reach underwater without any equipment, such as scuba gear or submarines, depends on various factors, including the individual's physical condition and training, as well as the effects of water pressure on the body.
When a person descends underwater, the pressure increases with depth due to the weight of the water above them. Water is much denser than air, so the pressure increases rapidly as you go deeper. The increase in pressure affects the body, primarily the lungs and the air spaces within the body.
The human body is not adapted to withstand the high pressures experienced at significant depths. As a result, the most significant danger of diving deep without equipment is the risk of barotrauma, which is damage caused by the pressure imbalance between the body's air spaces and the surrounding water.
At relatively shallow depths, the pressure is still manageable for the human body. Most recreational scuba divers, for example, are trained to dive up to a maximum depth of around 40 meters (130 feet). However, diving without any equipment makes it even more challenging to withstand increasing pressure as you go deeper.
If we consider unassisted free diving, where a person holds their breath and descends without the aid of breathing apparatus, the current world record for free diving in the "Constant Weight" category is around 128 meters (420 feet). This record is held by Herbert Nitsch, achieved in 2012.
It's important to note that free diving to such depths is an extremely dangerous activity and should only be attempted by highly trained individuals under controlled conditions. Without proper training, equipment, and safety protocols, descending to great depths underwater can lead to severe injury, drowning, or even death.
For safe and extended exploration of deeper depths, specialized equipment like submarines, diving suits, and breathing apparatus are necessary to withstand the increased pressure and maintain a supply of breathable air.