No interactions present.
You're right, it's a Grass Snake. It may not be cooling off though; they spend a fair bit of time in the water. I even saw a pair mating in the water about three years ago. They swim really quite well - one swam in front of me at Fowlmere on Monday.
Yes you are right. It was a good swimmer. The cooling off idea was probably projection on my part!
Lat/Lng: 52.1, 0.0
OS grid ref: TL4152