I'm hoping that it's a jellyfish. Here is what I hope it will be like:
It would basically look like a white squid with a shortened body. It would spawn uncommonly in light levels 9 and below. It would deal 3 hearts of contact damage per hit. It would have 8 hearts. I have multiple ideas for a drop:
1) It could drop jelly, which would be a slime ball substitute, but could not be crafted into blocks.
2) It could drop tentacles, which could be used to make blocks that damage you on contact, weapons, or poison potions.
I highly doubt it's kraken becuase we have 2 super famous mods that bring the kraken and since they already attract a ton of minecrafters I highly doubt jeb will see a need to make a kraken boss/mob into vanilla im kinda agreeing with the sea urchin idea it's unique and I have yet to see a popular mod to have already implimented the idea it will add a ton of uniqueness to our beloved game
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.
TIL that dolphins are disgusting little creatures.
I'd personally like a new unique mob like Creepers, Ghasts and Withers (Just realized that the main 3 unique mobs are also all explosive :P) Maybe something inspired by ancient marine reptiles like mosasaurs and pliosaurs, or deep sea life like the Angler Fish?