As an ocean lover the first thing that comes into my mind when I think about it is whales. Whales are that species that you would want to be simulated into Minecraft. Whales in Minecraft would be in families, you would have the mother, father and of course a baby whale! They sing together in a heart warming song.
Stats for the whales:
Health: Whales are tough creatures, the amount of damage to kill it would be a lot! I mean, why would you kill a whale anyway! There health pool would be so large that it would take around 3 - 4 minutes of you trying to take this big creature down.
Damage: The damage wouldn't be a lot, but still dangerous. When whales feel threatened they can slap their predators with their giant tail and cause knock back which would send them far away. It would also damage boats sending you in the sky!
Special: While whales are singing they give you extra health in the water for some time. Whales would also give you peace, a new special ability just from whales. Peace will give you the ability of Mobs that are dangerous to ignore you for some time while traveling in the ocean. That means husk will leave you only too!
I'll give you this that sounds really cool, but what about a squid as like a underwater boss that you have to summon similar to how the wither works bit differently like it can only be spawned underwater at a good depth and drop an exclusive item that can only be obtained by killing it