Garmin watches have some measures to compensate for this, although it only works relatively well when swimming freestyle. I have a Fenix 7 and it is remarkably accurate when swimming freestyle in open waters.
I knew about the problem, so last summer when I had a 1.5km openwater swim (*** style) I tried to mitigate it, but with not much success:
1. made sure GPS fix was OK before I entered the water and hit start
2. in every 200-ish meters I tried to raise my left wrist to be above the water for a few seconds. Maybe this wasn't enough, maybe it was a stupid idea from the start.
