Recently took a trip up to Maine and when walking near a river I noticed a grassy flooded area that was filled with lots of small aquatic insects. They were probably ~1/4”-1/2”, slender, light brown and very active, swimming gracefully under the water looking a lot like small fish. The water was very cold - some areas were mostly frozen over and I could still see the little guys swimming around under the ice. Any ideas what these might have been? Maybe some sort of diving beetle larvae?