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 a little extra weight in salt water compared to fresh water. This will be discussed in more detail in the next level of training.
While freediving (with no scuba unit) on the surface, you take a breath at 1 bar/ata and hold it. While descending, the increased water pressure compresses the air in your lungs. When you ascend, the air expands to the same volume that you originally had—1 bar/ata. You can hold your breath while freediving with no scuba unit. You are not breathing compressed air at depth.
Scuba diving is very different. Underwater, you breathe air at a pressure equal to the surrounding water pressure. Therefore, your lungs will always be at their normal volume at any depth. Refer to Table 1 below. If you hold your breath on ascent, your 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 very easy to avoid!
Therefore, the most important rule in scuba diving is to never hold your breath and breathe continuously.