I heard it was a submarine missle shot from either North Korean or George Soros
But seriously North Korea makes money selling miniature submarines to all sorts of folks. All they need is a "tender" offshore that looks like a fishing boat.
The trick is not to dive to the bottom of the ocean and blow something up ~ it's just to get underwater deep enough to evade ordinary detection then get close enough to your target to plant a remote controlled mine or IED.
The Coast Guard and Navy should be out there checking out the approaches to that rig ~ as recorded by our spy satellites.