There are other insects (and common outside Australia) that do this: Specifically, water boatmen which live underwater and have little legs that they use to push themselves around with. This includes the "lesser boatman", Corixidae and Pleidae, and the "greater boatman", Notonectidae.
Notonecta glauca is my favourite animal. It is a fearsome predator that can swim, walk and fly. Also, one of the few animals that can take-off from underwater!
In particular, Notonecta glauca (or common backswimmer) swims on its back and will walk under the water surface searching for prey: https://www.researchgate.net/figure/Lateral-view-on-the-wate....