I was excited to come across this little creature trying to dart around and stay under branches and leaf cover in the shallows - it's a predaceous diving beetle, sometimes referred to as a 'water tiger' because they are voracious predators, even in their larval stage. In turn, they make great snacks for fish! I believe this is the North American Acilius mediatus.
They have long hairs on their back legs like tassels, to help with swimming, and are able to breathe underwater by using the oxygen from the air bubble trapped under their wing cases, and which is absorbed through their spiracles - the original scuba tank, carrying a bubble of oxygen down under the water with them. I do believe one can see the outline of part of the air bubble on its back end on the pic above.
They're water beetles, so can swim and fly.