When I was in Ipswich Haven (also locked) there would be the occasional seal, even a porpoise once. they eventually find their way out on free flow which is fine if there are enough mullet or whatever in the marina to eat. Actually, there could be some fairly unusual wildlife in the marina, I was looking into the water once (like you do) and spotted a pipefish which is a bit like a straightened out seahorse!
Saw a bloody great seal in Fox's (where I am now) once which left quite an impressive bow wave/wash as it chased the fish. Normal sized seals in the open to river marina are fairly common.