Aquatic insect comprising 6 subfamilies and 55 genera. (The Wikipedia page's right-hand panel contradicts the detail below that).
Size 2 - 15 mm long (0.1 - 0.6 inches), six legs, the front pair are hairy and shaped like oars. Feeding on aquatic plants and algae they range worldwide except Antarctica.
If I couldn't guess from the picture which of the 55 genera it might be as so many are wide-ranging and have similar morphology.
Edit: in response to bob1's comment, the related family notonectidae (backswimmer) deserves a mention.
Being of the same order as the corixidae above (the neomorpha), the backswimmer is distinguished by ... swimming on it's back. They're slightly bigger on the whole than their cousins and although primarily aquatic are able to migrate through flight. They can inflict a painful bite and are able to pierce with their proboscis - which they use to great effect in hunting tadpoles and small fish.