But the Orion P3 is getting pingbacks, so it must be more than water. It could be a seal, a porpoise, a large fish such as a tuna, a whale or a shipping container. From our discussion, a 777 part would likely have sunk, so I'm from Missouri on that theory.
Kinda similar...(from a Tomnod map).
Wonder if the U.S. P-8 Poseidon & the Australian P-3 are using surface radar or may have dropped a sonobuoy?