Here is my 2 favourite "sea-spiracys"

The 52 hertz whale / the loney Whale / 52 Blue

There has been multiple underwater audio recordings (at 52 hertz) from all over the northern hemisphere, of what sounds to be Whale song. (This is fact)

Only problem is, for a Whale to make a call at 52 hertz it would have to be significantly larger than the biggest adult blue whales - almost 50% larger in fact.

The conspiracy theory is that a lone (possibly last of its kind) Whale has been swimming around the seas for a few decades searching for a mate.

It's call was first detected in 1989. As of 2014 it's been detected every year. It's still out there, we still think it's entirely possible it is an unknown Whale species.

Though it's most likely some kind of tectonic movement causing the sound.

The great shark killer.

In the late 80s, early 90s there was a project in Australia to tag and track Great White Sharks. One such shark, an adult female, was successfully tagged.

A week later her dorsal fin (with tag attatched) washed up on a beach. Researchers went out to find it, assuming she had beached herself, and were shocked to find her dorsal fin and section of her back had been ripped off. Large teeth marks (bigger than anything we know of in the ocean) were visably the cause of the severation.

Scientists then looked into the tracking data they got from her tag and found a truly shocking and scary discovery.

The shark had left her nursing pool to feed, leaving her pups in the shallows while she went out to deeper waters to hunt. There was an intercontinental shelf (underwater cliff) not far from the nursing grounds she was raising her babies in.

The tracker shows she fairly slowly swam out to the shelf, hung around for a few minutes looking for prey, then very suddenly and quickly - dived down, deep. Much deeper than a great white is capable of diving and stayed there for a few hours. It's not possible she could have done this herself.

When she came back up she followed currents that took her into land, and that's where her fin was found.

So basically, something came up from the deep and took her, whole! Only to later spit out the dorsal fin and tracker.

Over the years since this happened there has been several other sharks fins found around the same stretch of beach in similar conditions.

Isreal Shark Spy's

In a controversial theory that made international headlines, bloody shark attacks in Egypt were blamed on the Israeli spy agency. The 2010 Sharm El Sheikh shark attacks, which led to the injury of Russian and Ukrainian tourists and the death of a German woman, were unprecedented. But after the attack, the narrative shifted, with some theorists blaming the shark attacks on the Mossad agency.

Conspiracy theorists claimed that Mossad used GPS tracking devices to guide sharks into Egyptian waters. With the historical tension between Egypt and Israel, the conspiracy theory had enough fodder to fuel it.

"From sea, to sea, to under the sea"