Recently I found a few ocean mods for Minecraft. My favourite one is called better animals plus and it basically just adds over 30 types of animals to the game. In the Ocean it adds Nautiluses, Horse shoe crabs, Crabs, Flying fish, Jelly fish, Octopuses, Walruses, Eel, Sharks, Barracudas, Giant squids, Piranhas and Whales.
They are still constantly adding new animals and sea creatures. It really makes the game much more fun. With all the animals you can make actual zoos and aquarium. Sadly none of these hostile mobs will be added to Minecraft.
We might get Crabs and whales but Mojang has said that they will never add hostile mobs if they aren’t fantasy. Even though we have giant spiders and hostile silverfish I guess the size and the unlikeliness of a silverfish attacking you makes the real world animals a fantasy creature.
I will probably make a video for this mod soon.