a leviathan can attack submerged ships.
I would clarify what this means – can shoot at submerged ships, can ram them, etc.
After winning a boarding action
I would change it to “boarding party”, but that’s more of just my opinion on it.
Does 30 points for the example leviathan feel fair or still too cheap for what the keyword can do as-is?
With the updated version, I think it could work now. I was most concerned about the possibility of getting pinned and having almost no recourse for it.