I'll start from the treasure chest. Hookshot north and head through the door. Head left, pick up the pot by the door and push the statue onto the switch you'll find there. Don't go through the door, go right from the initial entrance and head through the door, NOT down the stairs. Follow the small passageway down a different flight of stairs. Push the lever to your left to drain the pool and go down the stairs into the pool's door. Head up, to where I'm guessing you're stuck. Go to the right of the room and walk into the second waterfall from the right, which hides an open door. Follow the simple progression of rooms until you reach a large deep pool which flows strongly east. Use a medallion to kill the creatures there, then jump in and swim to the far eastern door. Go up through the next door to meet the boss.