I don’t have an Apple Watch, but I do have a Garmin watch which measures Pulse Ox , blood oxygen levels. I find it to be unreliable in this respect, under reading by several percent. In fact so much so, I bought a Pulse oximeter unit (about £35) as one of the symptoms of Covid is low O2 in the blood. This seems to be more reliable- I have been tested by HCPs and their readings show I do not have a problem.
Perhaps get a cheaper version, and get a Pulse oximeter?