Semi-aquatic ichneumonid? Possibly undescribed.
I recently (May 1 to be precise) went to Barton Creek Habitat Preserve in Austin, Texas as part of a vertebrate natural history class I was leading. One of the coolest things I saw, however, was an invertebrate, a wasp that was crawling around underwater. It looked like a female ichneumonid that was searching for hosts. It was in very shallow, fast-flowing water where there were many black fly larvae (along with a few pupae). There were also mayfly nymphs and Hydropsychid caddisfly larval nets.
I did not collect it or take any photos, but I did record a video with my phone (unfortunately you will have to click the gear at the bottom of the video and change it to HD to really see much):
The wasp was around 7-9 mm. After a little research, it may be in Ichneumonidae, subfamily Orthocentrinae if it was after the blackflies, since that was the only subfamily with hosts in Nematocera I was able to find. Regardless, it sounds like there is a lot of undescribed diversity in semi-aquatic ichneumons, so it could be new.
I ended up seeing two, but I wasn't able to spend much time since I was leading a class about vertebrates. I'm returning to the same spot this Thursday for another class, and I may try to collect one then if they are still around. I'm sure there's someone out there who can give some context to this observation.