I found this beetle in my small garden pond and made some pictures (slides) of it. Not knowing then this would render a new impulse to my "ditch photography" passion, ending in spending a lot of time on this website.
By the grooves in the wing cases (elytra) you can recognise the female of the Great diving beetle. The males have smooth elytra and broadened front legs with sucking discs. Sometimes female specimen are found with smooth elytra, but never with sucking discs. Dytiscus marginalis is one of the largest beetles in the Northern hemisphere (and part of the Southern as well). It is a hungry carnivore that will bite in anything that is or was an animal. Despite (or thanks to?) that it is an interesting creature that can be easily held separate for a few days in a tank that is not to small. For food it will accept rainworms, flies, maggots, cat feed, etc. The Great diving beetle is often scooped out of garden ponds to be killed, because they may damage goldfish or expensive other fish. That really is pity, because it's natural habitats are almost completely destroyed. Better bring them in a ditch nearby or simply let them fly away. You could consider creating a small, more natural pond in your garden. You might find that the frogs, newts and beetles that come to visit it, are much more interesting then the eternal goldfish...