@malcyway
I don’t know the S9B at all, but what I understand (I recently missed it to setup correctly in my A3P and then could not set it anymore from MC app) is that if you have an activity with changing altitudes (skiing, hiking, cycling, mountaineering) you must use altimeter for the profile. If you go sailing, swimming or cycling in the netherlands you must use baro in the profile. The watch then knows that the altitude is fixed.
With swimming (or in my case, surfboard paddling) it is not optimal for the pressure sensors recordings as every stroke you make is once up in fresh air and once maybe up to 0.5m below the water and there is a significant pressure difference.