I don't think it's going to be a jellyfish, they don't really seem like a "nasty hostile mob", they kinda just float around. And it's not going to be a new squid variant (kids shouting Kraken, sit down) when squids are like the only mob in the ocean.
Hostile dolphin seems kind of weird, but it is true in real life, more people are raped to death by dolphins than eaten by sharks. With that in mind, however, adding hostile dolphins would be nothing short of a rape joke, which wouldn't belong in a game like Minecraft.
Personally, I think/hope that it's either a barracuda or some kind of fictional mob, maybe based on a prehistoric sea reptile (like a mesosaur or icthyosaur, but maybe a little smaller).
Also, ADD FISH AS A MOB! Jeb pls. With this new hostile sea creature, you could make it eat fish.
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Timeless Modding is kinda dead but idk what to put here now
Here's hoping it's something that when you're out in the ocean, music slowly starts, then as it gets nearer to you the music intensifies, then it scuttles your boat and you're trapped in the ocean as a giant mouth comes from the depths and eats you whole. only way to avoid it would be to get to land.
The real-life deep underwaters here on earth house some of the freakiest stuff we'll ever see, if Jeb will take any inspiration from there I may need to man myself up whenever I need to go diving in-game.