Fenix 6s Open Water Swimming under reporting distance & pace

I've upgraded to the Fenix 6s sapphire and the distance (and thus pace) is being under calculated on open water swimming. At first I thought I was just having a slow day or the current was stronger than usual but over multiple attempts I am getting the same result.

I have swam in the same bay for years and from my previous watch, triathlon times  and pool pace I know I average between 1:50 and 2:00 per 100m yet on the Garmin 6s it is showing more like 2:30m per 100m. 

I wore my friends Fenix 5 on one wrist with the 6 on the other and over a 600m swim the 6s reports 500m. Likewise over an Olympic triathlon distance swim (1800m) the 6s shows closer to 1500m. 

I have switched between glonass / galileo and changed GPS to every second to no avail. It seems to be more an error in the distance calculation rather than the GPS tracking as the route shows as accurate when looking at the  map post swim. Worth noting distance and pace is accurate for running and cycling. 

Has anyone else experienced this?