Apple Watch not measuring swimming correctly sometimes

Hello, I've noticed that my and also my mom's Apple Watch (I have Apple Watch SE 1 and my mom has Apple Watch 9) don't measure open water swimming in the sea correctly with native Workout app. We swam like 390 meters and my Apple Watch measured 60-90 meters and my mom's measured 56 meters. We needed to use pool swimming so it measures correctly. For me, it's working good, but yesterday, my mom swam like 800 meters and her Apple Watch measured only 120 meters even when using pool swim. Does anybody have this experience as well?