Sounding like something from Victorian London on the River Thames in fact a Water Boatman or a the Common Backswimmer is a water bug. It will swim upside-down through the water, They will grab insects that have fallen into the water. It is also an active and voracious predator, hunting smaller invertebrates, tadpoles and small fish. It will stab them with its ‘beak’, injecting a toxic saliva into the wound sucking out the contents of the body of its catch. You need to take care if you handle this bug as they can inflict a nasty bite!

Top view,

Underside view.
2 extended legs looking like oars these propel the boatman through the water, Its body resembles the shape of a boat. It may have a silvery appearance due to trapped air bubbles on its lower surface, which allow it to breathe.