@Mff73 you would loose altitude accuracy.
Reaching down to 20m under water results in 1bar additional pressure.
1bar=1000hpa
Around sea level 1hpa=8m of altitude.
So the factor of pressure change above and below sea level is 1:8000

Cannot figure out the rest of the math, but altitude would jump in like big steps and not the 1m we are used to.