Now, you know that salt water is denser (weighs more) than fresh water. So, to get the same pressure in fresh water as salt water, you need to descend just a little more. This means you also need to carry a little extra lead weight (ballast) in salt water compared to fresh water to overcome the increased buoyant force. This will be discussed in more detail in the next level of training.
While freediving (with no scuba unit) on the surface, the swimmer takes a breath at 1 bar/ata on the surface and holds it. While descending, the increased water pressure compresses the air in the diver's lungs. When they ascend, the air expands to its original volume—1 bar/ata. And so, it follows that a freediver can hold his or her breath while freediving with no scuba unit because the freediver is not breathing compressed air at depth.
Scuba diving is very different. Underwater, divers breathe air at a pressure equal to the surrounding ambient pressure (atmospheric and water pressure). Therefore, their lungs will always be at their normal volume at any depth. Refer to Table 1 again below. If divers hold their breaths on ascent, their lungs would overexpand and likely rupture. Lung overexpansion can force air into the bloodstream and chest cavity, which can lead to serious injuries, paralysis, and even death. Lung injuries are very difficult to treat, but we can greatly reduce the risk of them happening!
Therefore, the most important rule in scuba diving is to never hold your breath and to breathe continuously.