There are about 300 British water mites, and over 900 European species. Identification to species level is very difficult, and usually requires a microscope and complicated keys.
Above is Hydrarachna sp., a water mite.
Note that the first pair of legs do not have the swimming hairs that the other legs have. It is 5 - 8 mm long.
Hydrachna insert their eggs into water plants using their ovipositors.
Above is Hygrobates sp.
In contrast to Hydrarachna above it has no swimming hairs on its legs, just bristles.
It is common in weedy ponds especially among duckweed. It is yellowish-green or blue with dark markings.