Predacious Diving Beetle, Family Dytiscidae, i think Colymbeters sculptilis
"We dug a pond next to the alder trees last week and already it's filled with water beetles. I don't know where they came from. Nature's ability to rejuvenate itself, to be born and born and born again, never ceases to come in at me when I least expect it." ~ Paul Kingsnorth, "The Keyboard and the Spade", 30 December, 2015.