So, I have commented previously on this matter. It really bugs me ! I've swum 285000m in the Irish sea over the last year. I used to think the 935 was fairly accurate until I started swimming a 320m triangle course. Each time I do that, the 935 underestimates by 20%. The track shows truncated triangles. I compared the track with a Locosys BT 31 in a swim buoy. I was astonished that over a 23 minute swim the 935 only recorded about 25 data points. The Locosys recorded over 2000 points Garmin boast that the 935 has an algorithm that takes account of the break in gps signal each time you put your wrist in the water. It doesn't seem to work.
Straight line swimming fairs OK because you only need 3 points to be recorded: start, turn around and end data points, to get accurate distance and therefore pace etc.
I contacted Garmin who gave me a replacement unit. Same issue.
I've tried all the different GPS settings. Updates, blah blah. Maybe the new Apple watch will be better. I'm really disappointed that a €550 Garmin is so inaccurate. 
