So I'm starting school for my Bachelor's in Programming and Game dev in a few weeks. This has made my creative side pop, so I've been coming up with ideas. This is just a suggestion that I thought would work well and be interesting in SoT. This one is very Percy Jackson sea monster based.
Charybdis would work very well in the Sea of Thieves atmosphere. This would be an opportunity to add Maelstroms, a new AI/boss raid, and a way to bring players together to defeat this beast. The Charybdis doesn't even have to be fully modeled out of water. A maelstrom could be present which could completely be made of the Charybdis mouth. Any ships in the area slow down to the speed similar or the same to the Kraken fight so you don't immediately get sucked into the mouth.
Around the edge of the Maelstrom would be more teath that damage your ship if you ram them, just like small islands. To defeat this beast you would have to blow up his insides! Drop gunpowder barrels into the water, which will travel into the middle of the Maelstrom (Charybdis mouth). Even Kraken tentacles around the edge would work, not needed though.
This is just a suggestion I came up with. Let me know what you think!