Pond dipping at the weekend with my daughter, we found a lot of beetles, some large, some small. It turns out there are a lot of species of dving beetle, so I can't be very sure what this fairly large one is, but my best guess is Dytiscus semisulcatus or similar.

It's at the surface here, filling up the air bubble under its elytra (wing cases) so that it can dive again. It's about 18mm long and swims powerfully with the rear legs that you can see are swept forwards in the photo above. Interestingly I've only ever found one or two small beetles in this same spot before, so I was pleased to come across so many this time, and I hope it's indicative of improving habitat.