First spotted in May 2010, this looks rather like the Water Boatman which swims on its back, but as it is swimming the right way up it is a Lesser Boatman.

It was seen to be coming to the surface, presumably to breath, and then diving into the murky water at the side of the pond.

Feeds on algae and plant debris, it flies, which is presumably how it got to the pond.

This is a bug, not a beetle, because its wings overlap under the wing cases, and it has sucking rather than biting mouth parts which beetles have, and beetles have wings that meet symetrically down the back.