Open water swimming

Hello

I tried swim in open water. 
I turn on swim in open water activity, after this I see the screen with time and distance. I stay, not swim but distance increases.  I were staying for 3min. but distance increased by 50 meters. What is the reason of such behavior?
If I use running activity and stand still I can see that distance changes but only by 0.04 meters.

I used GPS+GLONASS mode.