Prey of Phidippus princeps
Maybe the most amazing thing I've ever seen -- just wish I'd taken better pictures.
The wasp was using its wings to generate the forward momentum used to haul the spider across the surface of the water. It traveled much faster than I imagined from the pictures I've seen previously, and I likely would not have seen it at all if it hadn't been for the impressive wake it generated.
The habitat is a shallow wetland lying between occasionally flooded woods. There is a lot of Persicaria and Saururus around the margins, with a few groupings of Pontederia intermixed. A healthy population of duckweed and water-primrose covers the surface in the shallower areas. The wasp was traversing the deepest channel, heading towards the woods beyond a screen of buttonbush.
The prey appears to be Dolomedes, likely triton. I have created a separate observation for it, here: https://www.inaturalist.org/observations/245884144