I went on a walk to see what species of damselflies are out at the moment. I didn't see many, may be because of the weather, just these two:
A large red damselfly (Pyrrhosoma nymphula):
Olympus Stylus 1s, 300mm, ISO160, f5.6, 1/500s
A female of unknown species depositing eggs:
Olympus Stylus 1s, 300mm, ISO200, f5.6, 1/200s
Then, I noticed a strange bug swimming along. Just as I was getting used to insects and spiders walking on water, this one was swimming upside-down just below the surface:
Olympus Stylus 1s, 300mm, ISO200, f5.6, 1/125s
There was a bit of glare from the water, and I didn't bring a polarisation filter, so I took some extra photos to maybe get a better look, here's one:
Olympus Stylus 1s, 300mm, ISO200, f5.6, 1/125s
It looked like it was walking on the underside of the water surface.
I looked it up and it turns out to be one of the backswimmers (Notonectidae), probably a common backswimmer (Notonecta glauca), bootsmannetje in Dutch.
Finding its name didn't help; I still think it's a weird bug, but that feeling could well be mutual.