What Lies Below the Waves of Violent Blue?

What lies below the waves of violent blue?

Away from sunlight in a scary gloom. 

A man's worst fears could actually be true,

A monster sealing every fish's doom.

It's mouth row of terrifying teeth,

Its scales are like polished jewels, though none can see.

Below the tide, it does not need to breathe,

Away from fishing hooks, it feels so free.

The thing, unheard, sneaks up behind its prey

He swallows each one up with speed and ease.

No fish can hide; no fish can swim away,

He finds each one as swiftly as the breeze.

And now I dare not swim in fear of it,

But on the beach, in safety, I will sit.