Open water swimming incorrect distance despite correct geolocation

When i'm recording an open water swimming activity the watch keeps increasing the distance despite staying in one place. It just seems to increase linearily, no matter what i do. It doesn't seem to be a GPS issue, since it doesn't change if i hold my hand above or below the water (and GPS is actually pretty stable). It's like the watch is trying to extrapolate from my previus movement, despite available accurate GPS data. This mostly happens when I take a rest after swimming a few hundred meters.

When i looked at the activity on the map and estimated the distance from that, the distance was more than 2 times more.

After i've looked at the pace graphs, it seems like there is a constant movement being added to the data for some reason (see attached image), whenever i stay in one place even for a tiny bit. You can even see that i wasn't swimming on my heart rate and yet the watch thinks i was swimming faster than ever.

I don't know why exactly this happens, but it seems like something that was deliberately added and not a glitch. Some extrapolation technic gone wrong? This makes this type of the activity completely unusable for me, since i am interested in how much i actually swam. I am better off using the running or walking activity for that, which kind of defeats the purpose of having a dedicated activity for swimming in the 1st place. 

PS. The way images are uploaded on this website is kinda silly, i've just deleted a paragraph of text by dragging the image file into the editor. And the picture is all low res. Seems like some people never heard of the browse button.