Swim across to the area you couldn't reach before. Get past the spike trap and kill the skimming insects, avoiding the ball of water that bounces around, to make a chest appear containing a key. Get the key and go through the locked door, down the stairs.
Careful of the skimmers, ball of water, and fireball spewing statue in here. Bomb the wall here for the map; Zols will pop out of the ground behind you, so kill those and get back to the last room.
Go south and pick up the bottom skull to find a key. Use it on the locked southwest door.
The yellow thing in the center will shoot a fireball in your direction whenever you swing your sword, so don't stand in its path while you're fighting. Get past the pink-skulled stalfos and red Bari, and go down the stairs. Exit through the north door on the lower level. Pick up the skull for a key and go back to the last room. Go up the stairs to the left and go through the locked door.
Kill the stalfos or avoid them, and pound down the pink things to get to the switch. Get behind it (to the right side of it) and push, and water will pour out of the statue's mouth. (Hopefully you got the key from under the skull before throwing the switch; otherwise you'll have to save and reset.) You can ignore the cracked wall to the left, it just leads to two skulls hiding five rupees and a bomb. Exit the room to the south.
Swim over to the short ladder to the left and jump onto the ledge. Exit through the west door.
The holes around the ceiling are home to evil fish; every few seconds, one will fall down from the hole and start swimming around the shallow water on the floor. You'll know that one is about to pop out when you see its glowing white eyes in the hole. Be careful not to let it land on you; it takes two hearts of damage! Once they land, they swim around randomly along the edges of the room, so watch out.
Avoid the fish, skimmers, and fireballs and go down the stairs or jump off the ledge into the shallow water, then exit through the southern door. Go east and south through the door, then around through the other door back to where you were, but on the other side of the blocks. Push the bottom block right and a chest will appear. Open it for the compass. Then exit this room through the north door.
Go up the stairs at the south-west of the room and through the door. Get past the monsters and go down the stairs (none of the blocks can be pushed, so don't bother), then through the door. In this familiar-looking room, lift the skull and get the key underneath, then pass through the last room and return to the large room with the falling fish.
Go up the north-west stairs and through the locked door. Hit the crystal switch through the wall (which your sword somehow passes through), and push the switch to fill the corridor with water. Get on the other side of the wall and hit the crystal switch again to make the blue blocks go up. Head east to the falling fish room.
Take the south-west exit once more, and now you can swim over to the west exit, so do that and go through. Avoiding the monsters, head downstairs. You have a block puzzle to solve; push the bottom block left, and the center block up to get past, go up the stairs, and up the stairs in the north wall.
Push the block out of the way and avoid all the many monsters and fireballs in this room to go to the north-east corner and push the block down or up. Fall down the hole here, then exit east to a room with a chest containing the big key. Go back to the last room and drop off the ledge to the shallow water below.
You can see a chest across the gap that you'd have to go back upstairs and down the left hole to get, but it just contains a red rupee. The room above is tough, so don't bother. Instead, from where you're standing in the shallow water where you dropped off the ledge, go south and up the right-hand stairs, then exit east to the large room again.
With the big key in hand, go up the stairs to the platform in the center of the room and open the big chest for this dungeon's treasure: the hookshot. This handy item can be shot straight out from you, and will grab onto a far away object and either bring it closer to you, or pull you across to it.
Still standing on the platform with the big chest, use your spankin' new hookshot on the skull on the ledge to the right of the big chest platform. Pick up the skull at the far right for a small key. Then get back on the big chest platform, walk behind the big chest, and hookshot one of the skulls on the north ledge. Go through the locked door.
Go to the left side of the room and pick up the skull in the top-left corner. A switch is underneath, but needs something on top of it to stay down. The nearby statue can be moved around, so move it onto the switch, BUT! don't go in the door that opens here. Instead, go through the open door (not the stairs) on the right side of the room.
Get the red Bari out of the way and go down the stairs in this oddly-shaped room. Below, you find a water switch. (If you had taken the other door, you would have ended up on the wrong side of it, see?) Push the switch, and a drain magically appears in this pool of water, allowing you access to the area below. Go down and through the door. If you would like two hearts and 40 rupees, go south and use the door below to find two skulls and two chests. Then in the room with the shallow water, go through the door at the top of the screen.
If you push the third block from the left, a chest will appear containing 20 rupees, so get that if you want. The open door in the north wall just has skulls with items and rupees, so if you need bombs and/or arrows, go there.
Back in the waterfall room, go through the second waterfall from the right to get through to another room with two blue Bari and stairs in the north wall. Go up.
After passing through the corridor with the blue Bari and hiding Zols, immediately jump into the water here, or the skimmers will gather near the edge and you won't be able to jump off safely. Swim east, against the current, and get up on the ledge with the skull. Pick it up for a small key. Then try to lure the skimmers to one side so you can safely jump off the ledge and swim west. The cracked wall hides skulls with hearts and other items. Go through the locked door. This is the boss door! Go through to meet...
-- Arrghus --
This boss is surrounded by puff balls and can't be harmed while they're there. Arrghus will cause the puff balls to expand and spin around the room, so watch out. Use your hookshot to pull the puff balls to you (be careful if you do this from below; the puff ball will end up on top of you and may hurt you) and then use your sword to destroy them one-by-one. Once all the puff balls are gone, Arrghus will jump high up to the ceiling. It will then attempt to land on you, so listen for the "falling" sound and get out of the way. Once it lands, it will start zooming diagonally around the room. Avoid it, and hit it with your sword. It will bounce up again and repeat the pattern. Keep hitting it with your sword while it's zooming around until it dies.
Step into the exploding mass of the defeated boss to collect the heart container it leaves behind, then stand in the middle of the room to catch the falling crystal.
The maiden inside said that the wishes of the person who touches the Triforce will last as long as that person lives. Ganondorf, the evil thief, rediscovered the knowledge of the location of the Triforce, but got stuck in the Dark World. There are other warps to the Dark World, like the one on Death Mountain, hidden around Hyrule. Your hearts and magic will be filled and you will be returned outside the dungeon!