Please see picture attached. I've looked at several sites on-line and couldn't find it anywhere. It is about 2cm long and I found it dead in a bucket of water. The bucket had been left unattended for about two weeks. I'm guessing it was looking for water in the dry weather and couldn't get out.
I have always regarded Dytiscus marginalis as the psychopath of the insect world as they will fearlessly attack just about anything that moves. As a child I saw one take a bite from the back of a five inch Chub in a local stream and in a weeded canal I saw one make short work of a huge Marsh Frog tadpole also to add to my ever increasing tally of nips I have personally been bitten a few times when delving into a weeded net, very bloody and painful.