In mid July I was photographing mud dauber wasps in the mud in the brook in the back yard when I spied this worm in the water. You can't see the water in the photo because it was so clear, but this larva is completely submerged and was on the bottom of the stream bed. It is the larva of a crane fly (also called a mosquito hawk). These can be quite the pest, apparently, because they eat the roots of grass and can ruin a lawn. We haven't had any such problem, though. From larva to adult crane fly is quite the metamorphosis.