No, just brain damaged from oxygen deprivation

They’re only down there for a very short time and they don’t have a lot of nitrogen stored in their lungs, so decompression sickness is usually not an issue. I think there have been a few cases, especially with repeated, very deep dives. But nothing you have to worry about as a normal hobby freediver.

Passing out at the surface is quite common though, but not due to decompression sickness. It’s the lack of oxygen that can happen when you have learned to completely ignore your urge to breathe and then stay down for too long. The reason why you pass out at the surface and almost never at the bottom, is because the water pressure compresses the air in your lungs. At a depth of 10 meters (30 feet), four liters of air in your lungs are compressed down to two liters. This is basically “concentrated air”, which contains “concentrated oxygen”. If the air is compressed to half the volume, it’s like having twice as much oxygen in it. Then, as you ascend to the surface, the air in your lungs expands again, turning the concentrated oxygen into regular oxygen and then it’s just not enough anymore and you pass out.

That’s why it’s recommended to always have someone with you who stays at at the surface and who can step in if you pass out, keeping your head above the water. It has never happened to me, but I’m really a beginner and I still have an urge to breathe that forces me to go back to the surface after a short while.

Nitrogen also becomes toxic, requiring you to substitute it with helium (or hydrogen).
Interesting talk on the subject: https://youtu.be/skL5EQa8DFY

“Dammit, monster! Get off my lawn! I ain’t giving you no tree-fitty!” It said, “How about just two-fitty?” I said, “Oh, now it’s only two-fitty!! What?! Is there a sale on Loch Ness munchies or something?!”

your mom and I 1.8288m’d last night

It’s less the result of a sensible system of units (like how 1 L of water ideally weighs 1 kg), and more fortunate happenstance in this case.

The formula for hydrostatic pressure* is:

∆P= ρ·g·∆h

where ∆P is the difference in pressure across the difference in height ∆h, ρ is the density of the liquid (~1000 kg/m³ for water, slightly more for sea water), and g is the acceleration due to gravity.

So the reason it works out nicely is because g is a little bit less than a nice factor of ten (9.8 m/s²), and the density of sea water is a little bit more than a nice factor of ten (typically 1025 kg/m³), and 1 atm also happens to be almost a nice factor of ten (101,325 Pa). That’s why the difference between the approximation and the actual* is less than a percent.

*This assumes a constant density of the liquid, which for water is reasonable, however different depths can have different salinities and temperatures in layers which change the density by less than a percent. Additionally, this assumes a constant acceleration due to gravity. At depth, the acceleration due to gravity can be higher, but this also has an effect that amounts to less than a percent even at the deepest point in the ocean.

ewww standard atmospheres. Use kilopascals like the good lord BIPM intended