How do you save the monkey in majora's mask?

You enter the room and roll into the pillar on the left or right with a treasure chest. Open the chest to find a key. You swim around the bridge. Best to dive into the right side and climb up on the last block. Then you go around the boxes and sometimes in the diagonals where they intersect and light the torches. Then you climb up the steps fight a huge spider and save your monkey by using the key you got and unlocking the monkey from its cage.