After reaching Tree Town, talk to a woman with an exclamation mark, then go into the building and talk to the Chief. This opens a minigame in the top left; clearing the minigame has you going back to the Chief, who gives you a
key Useless Object.
Head east out of Tree Town to the hatch you passed on the way in and use the
key Useless Object to open the hatch. Head down into the shaft and out of the bottom-right door, back into the Sunken Ruins. Talk to Uncle and he'll give you the valve.
Go back to the entrance and head left. Swim to the left, then use Newt Climb to reach the platform. Interact to raise the sea level and get the achievement.