Yesterday I swam a 28 km session and whe I finished I saw an improbable 32,7 km displayed activity.
When I analyze the track I see that it is well recorded by the GPS, a very clean track wit a too long calculated distanced. On Garmind Connect and on my Fenix 6 I see 32.715 mt but, after having exported the track in kml format and having imported it into Google Earth, the measured distance is instead a more truthful session from 27.767 mt.
In conclusion, the Fenix 6 (firmware 10.10, GPS 4.40) records the track well but the algorithm that calculates the distance does it incorrectly.
Garmin Connect activity: https://connect.garmin.com/modern/activity/5256840443
Fit file: https://drive.google.com/file/d/1_6JPkBthVOLW8PnuehvyAWMQwtBhO1rl/view?usp=sharing
Kml exportation: https://drive.google.com/file/d/1CXcj96BTKnxl1vcjC-fvCkSn_2IyIcxL/view?usp=sharing
