Slow worm behaviour

Been meaning to join the community for ages and eventually got round to it!

End of June this year, I saw the most bizarre behaviour by a slow worm. I was out walking alongside the local river and noticed something wriggling in the water on the far side. Had my zoom lens on the camera and could see it was a slow worm. I thought it was in difficulty but after watching it for a few minutes realised it was actively hunting in the a calm shallow area in the river (which was quite low at that time). Watched it for about 20 minutes altogether. There was plenty of opportunity for it to easily get out the river but it didn't. At times it would move its body slightly to enable it to stay stationary in the gentle flow of the river then would suddenly dart off swimming towards the water line of a rock and seemed to be picking at ???lavae (I couldn't see that much detail) at the waterline or stretching just above and occasionally ducking below the waterline to get whatever had caught its interest. Only lost sight of it after 20 minutes when it let itself be gently taken further down the river and rocks then occluded my view. Got a few poor quality photos as heavily cropped and ISO was at 8000 at some points as it was in quite heavy shade much of the time.

When I got home I googled swimming slow worms and quite few sites had people saying yes they can swim though don't do it by choice but this one most certainly was.