Backswimmer
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Notonecta maculata
Common & widespread in Britain.
Backswimmers swim on their backs, vigorously paddling with their long, hair-fringed hind legs and attack prey as large as tadpoles and small fish.
They can inflict a painful "bite" on a human being, actually a stab with their sharp tubular mouthparts (proboscis).
They inhabit still freshwater, e.g. lakes, ponds, marshes, and are sometimes found in garden ponds and even swimming pools.
Although primarily aquatic, they can fly well and so can disperse easily to new habitats.