For my first 2 swim activities, I found that the distance counted is often incorrect. Before switching to Garmin Forerunner 265 last week, I was using Apple Watch Ultra 2 and did not have this issue.
I swim in a 25 yard pool and do two lengths before taking a rest. I do not do a flip turn at the end of the first length. I would assume that this kind of pool swim would be easy to count. For 5 of my 30 length 2 laps, I found that the watch recorded the distance as 25m and for one it recorded as 75m. What could be going on here?