Can you say that a creature is really a "disguised sea serpent" if it's aquatic?
Well, maybe my grammar isn't ideal, but in truth I was borrowing the language from Yig's alteration of the Cultists.
"Yig's worshippers are actually disguised serpent people. Their bite is highly poisonous. Cultists have a combat rating of +0 and a combat damage of 4 Stamina."
(The underlining above is mine)
I needed to make Cultists aquatic and tougher so that there would be more (tough) aquatic monsters than normal and Leviathan's slumber effect would have a little bite to it.
By the way, we fought him last night. We were well on our way to a sealed gates victory when the Deep Ones Rising Track filled up. We failed in the Final (Epic) battle.
He has SO many hitpoints that he's very tough to take down, especially if you're not really stocked with magic weapons.