There are several dozen species of water snail in the UK. This one is the Pond Snail (or Great Pond Snail). They have a long pointed shell, and can be quite large - anything up to 7cm. These were around 2-3cm. They breath air, but can also get part of their oxygen underwater through their skin. When breathing air, they float upside-down on the surface. They can also move around by crawling along the surface in this way. They can no doubt be found anywhere in Croome's large lake, but shallow water at the edge is the best place to see them.