Spotted this beetle on Morfa Nefyn, a beach in North Wales, UK. The beach is a mixture of sand and stone, with large sandy cliff faces and rocky outcrops.
The beetle was about 3 to 5 mm or so long, running around on the rocks at the edge of a rock pool (not in the water though!), on a warm sunny day. It was moving are quickly, but not really fast, maybe around 1cm per second.
I've seen them on the beach before and sure I know what they were called but can't for the life of me remember.