Well Good News! This mod does exactly what you need, transform the seas into the most eerie and unsettling area in the game, even worse than caves.
Inspired by a monster from Monster Legends, this squid looking creature is a huge entity that lives in the seas and loves creeping players off. What's even better is that it doesn't kill you, instead it makes sure to keep you alive in order to mess with you even more (Unless...You find it's weakness).
DO NOT USE SOUND PHYSICS MOD
(it makes the sounds underwater way too low)
GECKOLIB NEEDED:
https://www.curseforge.com/minecraft/mc-mods/geckolib
Also Note that when making this mod, I had no initial knowledge of the actual Cthulhu seen online, which explains the size and model difference of what you may have been expecting
If you do encounter any bug or have any idea you want to see for the mod in the next updates, let me know in the comments and I'll consider having your feature implemented.
The model comes from Monster Legends