The numerous sea serpent skulls and skeletons around the isles makes me think they will be in the game. A sort of mini boss or creature seen swimming in the deep. Or to add danger to the treasure hunting, make them guard chests.
Sea Serpent Skeleton:
(https://goo.gl/images/1BqKsD)
I think only having the Kraken in the magical seas would be a somewhat injustice to the waves, They have a plethora of beasts at there hands
