30 meters is around the commonly accepted limit for recreational scuba diving.
If you are diving you can take a breath of air from your regulator at 30 meters and then, keeping your airway open, ascend to the surface on a single breath. Because you are ascending, the air in your lungs expands as you go up, and you get to the surface with a full breath of air, even though you've been blowing bubbles all the way up.
At 30 meters you limit your dive to 20 minutes... beyond that you risk decompression sickness.
At 10 meters the limit is around 3 hours.
In both cases, if you were down for a couple of days, you'd need decompression. The deeper you go, of course, the more dangerous it is as there is more dissolved gas.
If you are diving you can take a breath of air from your regulator at 30 meters and then, keeping your airway open, ascend to the surface on a single breath. Because you are ascending, the air in your lungs expands as you go up, and you get to the surface with a full breath of air, even though you've been blowing bubbles all the way up.
At 30 meters you limit your dive to 20 minutes... beyond that you risk decompression sickness. At 10 meters the limit is around 3 hours.
In both cases, if you were down for a couple of days, you'd need decompression. The deeper you go, of course, the more dangerous it is as there is more dissolved gas.