For example, once you reach the peaks of Mount Everest the air gets so thinned that you usually need oxygen masks/tanks so you stay alive.
Once you get to the various layers of our atmosphere it's virtually impossible to breathe without some sort of life-supporting equipment.
Mind you, Mount Everest at its peaks is around 8 km. As you can see in the image above, the troposphere, the layer closest to the Earth, goes up to 20 km. And the other layers go up to hundreds and thousands of kilometers. And when you have Goku fighting in the thermosphere or exosphere with Beerus for extended periods of time then the reasoning that he's still technically within the atmosphere makes no sense since there's no real air to breathe at those points.
Meaning if he can actually fight in those conditions then by default he should be able to move around in space or go underwater without worrying about breathing.