Pool Swim Inaccuracy
I used to swim with Huawei GT2 pro. It makes very accurate laps and distance counting. However, with new Apple Watch Ultra I found it misses 3-4 laps from 40. I have checked it a few times compering with Huawei and counting manually. The results were similar. My wife has Apple watch 5 and she has faced the same problem. It does not depend on swimming style. We use custom swimming pool size 19 meters.
I found old topic with the same question. And it looks like no fix yet.
Apple Watch Ultra