Ol' Plank Eater

No on listens to Ol' Peaches about Ol' Plank Eater, so why should I talk through this again? Maybe I'd prefer to save my breath, rather than talk about seamonsters that can swallow up ships whole if they choose it!

-Overheard in a seaside bar.

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There are certainly dangers to traveling amongst the clouds, that I will not deny, but at least within the embrace of sky the dragons are the dominant force and there are few, and I only say few because I assume there have to be, or at least have to have been, Dragons that subsisted off of human flesh. It is entirely possible, and even preferable that they only hunt monsters and other non-sentient creatures.
When one travels by sea, however, you can no longer be sure of such a fact. At the very least, much less so than with the dragons.
Stories persist over the centuries of the monsters that prowl the waves and below, and I do not wish to meet them if even half of the stories are true—Octopus tendrils the size of trains, maws that can bite a dreadnought in half. Many consider such tales as unsubstantiated, but there are far too many ships missing from the rolls for it to simply be bad chart readings.
In the Heartless Waves, the name "Ol' Plank Eater" has persisted since before the Dracanomachi as a warning towards those that wished to search for their fortunes out at sea. This particular story was of the "maw so large to bite a dreadnought in half" variety, and its list of supposed victims is a mile long.
From what I can gather, there are a few convergent points of description from those who say they've seen the beast.
A storm constantly accompanies the creature, and ships can find themselves caught in a storm for days on end when it's found its prey. From there, the ship will run aground, even as they sail through open ocean. An investigation will reveal perfectly round points sticking through the hull and keel. Massive things, large enough around to fight a full grown man inside.
Then the tearing starts.