Are we talking linear measurements or depth?
Depth is measured by pressure. That pressure is usually measured in fsw (feet of sea water) or msw (meters of sea water). There is an increase of 1 atm pressure when depth is increased by 33 fsw (or 10 msw) by definition. Therefore 20 msw exactly equals 66 fsw. Since 33 fsw does not exactly equal 33 linear feet from the surface (nor does 10 msw exactly equal 10 linear meters from the surface), the same cannot be said for linear measurement.
Depth is measured by pressure. That pressure is usually measured in fsw (feet of sea water) or msw (meters of sea water). There is an increase of 1 atm pressure when depth is increased by 33 fsw (or 10 msw) by definition. Therefore 20 msw exactly equals 66 fsw. Since 33 fsw does not exactly equal 33 linear feet from the surface (nor does 10 msw exactly equal 10 linear meters from the surface), the same cannot be said for linear measurement.