Here is the segment I found on that dungeon:
Once you defeat the two red zols in this room and a small key will
fall from the ceiling, but unfortunately, it falls right through the
holes in the ground. If we walk back to the right a screen and go down
the stairs we will see the key, but we cannot get it quite yet. So
instead, from where you defeated the zols, just head down a screen.
Speak with the owl statue and it gives you a hint about the glint of the tiles that we will find in one of the rooms ahead. Keep that in
mind as you avoid the enemies and head down a screen. Use roc’s
feather to jump over the deep water that is marked by the dark blue
color. Avoid the water tektites and peahats as you head left two
screens and up a screen. There is a treasure chest in this room, but
it is a dud and only contains a enemy zol inside. Avoid the enemies
and then head up a screen.
There are five strange tiles found on the ground in this room, but we cannot do much with them just yet. Just ignore them for now and
head right a screen. Open the treasure chest that is found in this
room to get a small key. With the small key in hand, we need to
backtrack left a screen, down two, right two, and up two to get back
to the room where we defeated two red zols and a small key dropped.
From here head up a screen to take on the dungeon mini-boss, Cueball.
Cueball can be confusing at first, but is really one of the easiest mini-bosses in the game. Its weak spot is its giant head and there are
multiple ways to go about attacking him. Cueball will simple move
around the room trying to dash into you. One way to defeat him is to
equip your Pegasus boots and dash around the room and eventually
strike him in the back of his head. Another way is to equip Roc’s
Feather and jump over Cueball and then deliver a sword slash. Perhaps
the easiest way is to stand near one of the corners of the room so you
are facing an area where Cueball has to change direction. Before he
gets a chance to change the way he’s moving, slash him at the side of
its body and you’ll deliver a blow. After a number of sword slashes,
Cueball will be defeated and the door above will open up, so head
~ Flippers ~
Equip the power bracelet and pull the lever back in this room as far as it goes to move the blocks apart. Quickly run to the left and
go through the door before the blocks close in. Watch out for the
fireballs that are shot towards you and defeat the red zol that are
found in this room. Open the treasure chest here to get the dungeon
item, the Flippers! These will now let Link swim in deep water as well
as underwater. They are automatically equipped as part of your
equipment and thus, they are not an equitable item. With our new
goodies in hand, head down a screen.
The new enemies here are known as iron masks and they can only be hit from behind. Use roc’s feather to jump over them and then quickly
deliver a blow to the back to defeat them. This is the room that the
owl statue was referring to earlier when it was talking about the
glint of the tiles. There is a glint on the middle tile and once you
walk over it, it moves to the tile at the bottom-left. What you need
to do is just walk over the five tiles in a row, without walking over
any of the other tiles. Once you’ve walked over all five of them the
door above will open up. More importantly though, we now know the
pattern of middle, bottom-left, top-right, top-left, and bottom-right.
With that in mind let’s head down a screen, push the block, and then
head left a screen.
~ Nightmare's Key ~
There are five tiles on the ground in this screen and you have to walk over them in that correct order, middle, bottom-left, top-right,
top-left, and bottom-right. (Order is random, can be different for
you) Once you’ve done so a staircase appears at the top-right part of
the screen so let’s head down it.
There is another Mario based enemy here known as a thwomp. Walk underneath it and it will slam to the ground, so quickly step back. As
it retracts to its original position, walk by it and continue to the
left. Trigger this second twomp and walk by this one as well. Trigger
it once more and then quickly climb the staircase, jump on top of it,
and then jump to the ladder to resurface.
Walk down a screen and open the treasure to get the nightmare’s key! With that goodie in hand let’s head up a screen, jump off the
ledge, and swim right two screens. Swim up a screen and open the
treasure chest found here to get a 50 rupees. With that in hand let’s
go right a screen and up a screen to get back to the room just before
> - If you haven’t already, defeat the red zols in this room to cause a small key to drop from the ceiling through one of the holes. Then walk
right a screen and head down the staircase. Jump over to the screen on
the left and then jump into the water to get the small key. Go back
right a screen and climb the steps to resurface.