backswimmer
noun
back·swim·mer ˈbak-ˌswi-mər 
: an aquatic bug (family Notonectidae) that swims on its back
Example Sentences
Recent Examples on the Web But these aquatic water scavenger beetles do not swim like the backswimmer or the larvae. 
New York Times, 26 July 2021 The 0.01-inch-long water flea is able to change its body into a bigger (and thus more inedible) shape when threatened by a predator, like the tiny backswimmer, says Anne Estes, a microbiologist at the University of Maryland School of Medicine. National Geographic, 12 Nov. 2016
New York Times, 26 July 2021 The 0.01-inch-long water flea is able to change its body into a bigger (and thus more inedible) shape when threatened by a predator, like the tiny backswimmer, says Anne Estes, a microbiologist at the University of Maryland School of Medicine. National Geographic, 12 Nov. 2016 Word History
First Known Use
1807, in the meaning defined above