When you get to the fourth warp room (the warp room available after you defeat Tiny), there will be a level called "Hangin' Out". Play through the level like normal, through a couple check points. You will come to a room with twelve boxes in it and a hole behind them. When you get to this room drop down the hole (Story progression and cannot be missed) and it will be the first time in this level that you land in water. There will be a checkpoint box straight ahead. Walk towards the screen away from the checkpoint box and there will be a small ledge to jump onto, then another hole you can jump into. Do this and it will bring you to a new area, complete the section and it will bring you back to the warp room and unlock the achievement.