I haven't noticed this, but if you look closely at the bottom of the map in the main menu of the game, you'll see a serpentine creature drawn next to a Megalodon near the drawn island that is shaped like Shark Bait Cove.
Could there actually be a third underwater threat? A Leviathan that is a type of large sea snake or eel? If so, we might get to fight something slightly bigger than a Meg.