... also because Sharks will naturally swim very close to land (litoral waters), at or near the surface of the water in addition to deeper waters further from the coast - all required for that essential surveillance role. Seeing a Tuna swimming just off a beach or within a harbour would immediately look out of place. Their characteristic fin projects above the water for extended periods, perfect for a camera to grab clear unobstructed shots.
A shark is large enough to carry all the equipment and batteries, but not so large that it can't slip through, or around anti-submarine and anti-torpedo nets.
They could have chosen a dolphin, but that is itself problematic as navies around the world have been training Dolphins to carry out surveillance and plant mines for decades. A lone dolphin swimming around your military vessels and ports would warrant close scrutiny.