I don't think rat snakes go into water, but may be wrong. I guess I've only known them to be around barns and such, hunting for critters in there. Moved square hay bale to feed horses in middle of winter, rat snake curled up under it, between bales. Gently replaced that bale, and it was funny what the hay stack looked like come spring! Used all bales around that part. LOL He was still there early spring, too, so he had a great place to hibernate. I have pics of my kids at ages probably 4 and 5, daughter holding a black rat snake that she had to hold UP to keep tail off the ground, and son laying down on the ground next to it. Early spring, it was not moving very fast. They begged me to catch it so they could touch it.
As a child, I caught one of those lime green snakes, (I always called garter snake, but not sure what they are called) and put it in my pocket. Got to friend's house, it was gone, had crawled out. So, went back to try to find it, and it had been run over on the road. Was soooo devastated!