Simple. Give it the same treatment you just gave temple. No one likes underwater combat. SO:
Move the underwater cap point to the deck of the boat. Move the boat up a bit on the map to maintain point distances. Sharks now just aggro anyone in the water.
This lets the cannon hit all 3 points, making it more important to hold. The center point on the boat would see a LOT of action. The water is now more a hazard to avoid or terrain to exploit than a place to fight.
I mean, who wouldn’t rather battle it out on the deck of a pirate ship above shark infested waters vs down fighting a ranger at Ruins all game?