Colossal Cave

You'll have a bottle of water at the beginning of the game. As you explore the caves, you'll come across a little plant growing underground. Give it the water and it'll grow a little. Exploring a little further (go between the plant and the wall, then follow the wall and enter a door; enter the next room, climb the wall with a rope and cross the bridge ), you'll come across an underground lake with a row boat near a pier. Walk up to the margin of the water and fill your bottle, then go back and use it on the plant again to grow it up and unlock the achievement.