In the field we thought these were caddisfly larvae but after later examination of photos, we weren't sure.
They were black, about 1 cm long, and they were swimming in groups, with several hundred in each group. (Photo was taken the following day.)
We identified these as wood frog eggs based on the photos in the manual and the egg ID quiz because there was no outer jelly matrix and the egg mass looked lumpy (and like bubbles at the water's surface).