To my eyes it strongly resembles a tire carcass. The proportions look odd though - with a relatively large wheel diameter, what most people call the "rim", compared to the tire. That is the opposite of the Lockheed tires, which had small diameter wheels relative to the tires.
Old tires are quite commonly used for boat bumpers, making it unsurprising to find one in a marine environment. They also come in a wide range of sizes.
Tire markings are molded in, and may be readible after long submersion.
I recall a discussion of a circular "wire" object in a different thread a couple years ago. One (of many) hypotheses at that time was that it might be the remnant of a tire bead.