When you jump into a hole, you will find a new area where you will be attacked by enemies. Kill them and then let BD-1 unlock the door for you. Go through that door until you reach some gap with water. Instead of monkey-bar your way over that gap take a dive instead and you will find a chest under the water. After opening the box, climb out of the water and use pipes to climb the other side.
When you reach the next room, turn off the electricity and swim to the other side. When you climb out of the water, you will notice an electric cable on your right hand. Cut it and swim back. Switch water turbine electricity back ON. Now look your left and you see a water turbine, stop it at the proper position with your force, go there and climb into a hole left side where you can find the Echo. After that swim over and take the elevator to the next floor.
