These are just a bunch of sea monsters I think would be cool to see in the game. Chances are none of these will be added, but it'd be cool nonetheless.
Aspidochelone
From Medieval folklore, Aspidochelone is a gigantic turtle (or sometimes whale). It's so large that it's often mistaken for a rocky, mountainous island. I think this would be cool to just have floating around with rare treasure on its back.
Iku-Turso
From Finnish folklore, Iku-Turso is a giant malevolent walrus (though the name now means 'octopus' in Finnish). He once set a bunch of hay on fire and from the ashes a tree grew so tall it cut the moon from the sky and blocked the sun.
Jörmungandr
From Norse mythology, Jörmungandr is an enormous sea serpent and is one of Loki's many children. As if being that big wasn't enough, his bites are also venomous.
Charybdis
From Greek mythology, believed to live under a rock across from another sea monster, Scylla. Charybdis would swallow huge amounts of water, creating whirlpools that could pull ships under.
Scylla
From Greek mythology, believed to live in a larger rock across from Charybdis. Once a beautiful nymph, she was transformed into a horrible monster. Tentacles, a cat tail, a belt of living dog heads around her waist and several long, hydra like heads extending past her own deformed head.