At my favourite watery wildlife spot I saw a new critter yesterday, though I knew what they were the minute I saw them: whirligig beetles. There are some great close-up photos and facts on that linked page - well worth a look. My photo above shows how metallic they appear on the surface - seedlike blobs of mercury almost.
I recognised them instantly because they were clearly beetles, and were whizzing around in circles and figures on eight on the surface in a fairly hectic manner. Particularly notable is that they have eyes split into two, for seeing above and below the water (the link above has a particularly good picture of this).