My Apple Watch Series 7 shows inconsistent heart rate readings when swimming

My series 7 Apple Watch rarely gives me more than three or four minutes of heart rate readings during my daily 1 K swim. even so, I refer to it because presumably if the heart rate is given towards the end of my swim, it is an accurate record of my highest exertion period. However, recently it has shown a heart rate of between 180 and 190 for much of the recorded time. It also fluctuates sometimes between 40 and 100, during my warm down. I was afraid that this was an episode of atrial fibrillation, which the watch has been good at catching, so I wore my wife’s watch on the other wrist to compare the data, and found that when one watch was fluctuating, the other watch showed a steadily declining heart rate after training, and vice versa. The measure is so inaccurate that I think I’ll buy a polar H 10 and compare the readings on both arms to try to get a more accurate image. Any suggestions?

[Re-Titled by Moderator]

Original Title: Heart rate record during swimming