Wrong Distance Swimming with Apple Watch SE (1st Gen.)

Hello!

My dad has an Apple Watch SE (Gen 1) paired with an iPhone 12. Last week he started to go to the swimming pool for a few laps, but the number of laps and km swam is extremely inaccurate. He is 70 and normally swims for about an hour doing 20 laps, while the Apple Watch counts it as being 44 or more.

This is too inaccurate, considering that this is the main reason he intends to really use the watch. I haven't found anyone experiencing this issue.

Both devices are up to date, and we have also reset fitness calibration data on the Watch and on the iPhone (and done the 30 mins outdoor walk).

Any ideas?