Confidence: I'm as sure as I can be.
Notes: It's an Isopod crustacean - Waterlouse. The body is flattened from top to bottom, and it has 7 pairs of walking legs. (Amphipods are flattened side to side). Need to see head pigmentation to decide which species, but 2 common ones are Asellus aquaticus and Proasellus meridianus (which tends not to occur in garden ponds).