This might be far fetched, but it would be interested if sirens were added into the oceans. Sirens and mermaids are usually mistaken for one another. The difference being a mermaid is half-human, half-fish, and the other being half-human, half-bird...
Weird concept, I know, but hear me out. It doesn't necessarily have to be a bird (that would look ridiculous). The mermaid concept is just fine but we would need to alter the image as to not confuse them with the mermaid saviors.
On stormy nights, or while passing cursed places, you'd see these apparitions come out of the water. Gorgeous beings with beautiful voices (male/ female or androgynous) that draw your eye. Literally. The players view is directed towards the sirens, with no choice of the player. They have to continuously redirect their gaze by shifting the mouse/stick in the direction they want to see. The closer they get to them, they will eventually start to drift towards them, and they'd constantly need to do their best to move away. If they fail to do so, they'll go overboard and are taken down to the depths.
If on land, the sirens have no effect, but I image them to be mainly sourced in the middle of the sea. It'd be funny if you had to play music and/or be drunk in order to stop the effect they have, since being drunk usually deafens your senses, and it still adds difficulty to maintaining the ship.
It sounds like a pain, but I feel like it'll add more excitement and difficulty. It'd be cool if they are put in locations with big loot, just so there is a reward. But it can be randomized like the Kraken and just another fun element of the game. It's an idea. Do with it what you will I guess.
