Circular fisheye lenses are fun and creative tools underwater. Unfortunately, for high optical quality they are expensive to buy and set up in an underwater housing. One lens that is affordable is by Inon Japan, and while it is not intended for circular images, with some creative zoom lenses this works fine - providing you are not seeking optical perfection. I actually like the inherent distortion in all fisheye lenses and with careful setup, composition and lighting, this Micro Fisheye from Inon a lot of fun to use underwater.
The intended use for this lens is as a close-up fisheye lens for compact camera setups, the idea being that you zoome the lens out to 80mm (35mm equivalent) and then have a very close-focussing suer wide angle view. It is sometimes referred to as a ‘bug-eye’ lens because of this extreme prespective and close focus distance, charmingly noted as 0cm in the specifications!