During mid May, I noticed lots of backswimmers in my tin bath pond. Since then they have diminished in numbers but grown in size, after several moults they are now about 1 cm long. They are Common Backswimmers, Notonecta glauca, one of the four British species. They swim on their backs, and come out to breath by touching the water surface with the tip of their abdomen. If you fish one out and put it in a small transparent container you might be able to see its white back by lifting the container and looking up. Mine have all white backs as they are not adults yet. They are predators of insects, which fall on the water and become trapped on the water surface, but as adults, they might catch small fish and tadpoles, and they are also cannibalistic. Avoid handling them, as they are said to be able to prick the skin with its mouth parts! They live in all sorts of ponds, including drinking troughs. They are winged and able to fly, and the adults can emerge out of water and disperse in search of new habitats.
This is my bath pond (photo from last year, the lily is not yet flowering this year).