Help on an animal I saw yesterday..?

Yesterday I went for my usual 30 minute walk. I live in the countryside and from our house to the main road there are country lanes and fords/rivers. Well when I walked down to the ford I walked onto the bridge and lent over to look across to the fields and when I looked down I saw something moving in the water. At first I thought it was a small log but realised it was crawling rather than drifting! So I went onto the road to see what it was. It was the strangest thing I had ever seen and looked almost like a lobster but half the size and brown. It crawled like one and had pincher's too. I live in England and near Warwickshire in the West Midlands so if anyone knows what the creature I just described was that would be great!