None of the above causes.
Its the mermen revolting and using their buddies to take out targets of opportunity - mermen are aquatic offshoot of human race from one of the human races aquatic stages hundreds of thousands of years ago. Them or aliens
Having hit whales and dolphins with ships, I can pretty much guarantee my posts #23, #38.
My surprise is the scientists couldn’t figure that out. But then I have seen where scientists discredit ocean phenomena expressed by sailors only to proclaim it years later as if they had discovered it.