Increase by one atmosphere. If it doubled then at the bottom of the Marianas Trench it'd be 10^330 atmospheres, or approximately 10^318 times greater than the pressure at the center of the Sun.
It doesn't double every 10m. Each 10m of salt water roughly adds 1 atm. So it only doubles at the first 10m. So the oxygen becomes toxic ~66 meters when diving with air.