After you find the Angler's Key in Yarna Desert, you can use it to unlock Angler's Tunnel in Tal Tal Heights.
Go north from the entrance, then go west. There are some flying bombs there. Use the Roc's Feather to jump up and get the bombs.
Get the Map
Go east from where you got the bombs, then go up the stairs. Go east, then keep going north until you reach a chest. Open it to get the Map.
Get a Key
Go south from where you got the map, then go down the stairs. Use the Pegasus Boots and Roc's Feather to jump over the gap in the middle of the room. Exit to the east.
Use a bomb on the cracked block, then push the other block into the water. Open the chest to get a key.
Get a Second Key
Go north until you reach a room with another chest. Use a bomb on the cracked block, then go up and open the chest to get another key.
Get the Stone Beak
Go south and then west, and jump over the gap. Go up the stairs, then go west, then south until you can go east. When you go east, you will reach a room with a chest. Open it to get the Stone Beak.
Get the Compass
Go west, then go down the stairs. Use your shield to flip over the Spiked Beetles. When you kill them, the door opens. Go east through it.
In the next room, open the chest to get the Compass.
Get a Third Key
Kill all the monsters in the Compass room to make the south door open. Go through.
Break the crystals with your sword to reach the chest. Open it to get a key.
Get a Fourth Key
Kill all the Water Tektites to open the door, then go north. Go east through the locked door. Use the Roc's Feather to jump over the deep water and make your way to the north door. Unlock it and go through.
Go north to the next area, then jump over the gap using both the Pegasus Boots and Roc's Feather. Then exit north.
Go north and unlock the key block. Push the block above it up. Then go west.
Kill the two Red Zols in this room to make a key fall from the ceiling. It falls down into a hole. Go south to the next room.
Go south to the next room, then use the Roc's Feather to jump along the shallow water and go west. Continue going west, then go north from the room with the Peahats.
There are two Stalfos in this room, and a chest. Don't open the chest, because it contains a Green Zol. Go north from this room.
There are five tiles in this room. For now, go east.
Open the chest to get a key.
Reach the Mini-Boss
Go west, south, south, east, east, and north to reach the room that has two holes in the floor and a locked door in the north wall. Go through that locked door.
Mini-Boss: Cue Ball
Cue Ball is only vulnerable from behind. You can use the Roc's Feather to jump over Cue Ball and hit it from behind. Try charging up a Spin Attack before you jump. Otherwise, if you are having trouble with that strategy, use the Pegasus Boots to dash around the room toward Cue Ball's back. You can hold the D-Pad while holding the Pegasus Boots button to have more control over your movement. When you hit Cue Ball, it might start going in the opposite direction, or it might continue to go in the same direction. After you hit Cue Ball enough times, it dies, leaving a fairy and a warp spot.
Go north to the next room.
Get the Flippers
Quickly go up to enter the room before the blocks move together and block the path. If you don't make it in time, just leave the room and go back in.
Go to the lever in the upper-right corner of the room. Equip the Power Bracelet and hold the lever, then go down to pull the lever downward. This will make the blocks move apart.
When the lever won't go any farther, use the Pegasus Boots to run to the west exit before the blocks close. The Spark is hard to avoid, so just do your best.
In the next room, kill the red Zols while avoiding the fireballs. When the Zols are gone, the fireballs stop, and the doors open. Open the treasure chest to get the Flippers. Now you can swim.
Get a Fifth Key
Use the south exit of the room where you got the Flippers.
Step on the sparkling tile. Another tile will start sparkling. Keep stepping on the sparkling tiles, making a note of the order that the tiles sparkle in. Or just read it below:
4 3 1 2 5
Afterward, go south, then swim east to the next room. Go east again and go down the stairs in the corner. Use the moving platform to reach the left side of the room. In the next room, get in the water and swim down to get the key.
Get 50 Rupees
You can optionally get 50 rupees. Go back the way you came until you are at the stairs that led to the underground area. Push the bottom-left block down, then go south, then use the Pegasus Boots and Roc's Feather to jump over the gap. Go south again.
Swim to the treasure chest and open it to get 50 rupees.
Then go north again and jump over the gap and go north to return to the room with the stairs in the corner. Push upward on the the block next to the bottom-left block.
Get the Nightmare's Key
Go west from the room with the stairs that are surrounded by ice tiles, then go west from the room with the two holes in the floor. Push the block to the left, then go through the west door. Step on the tiles in the order that you saw in the other room. Here it is again:
4 3 1 2 5
If you did it correctly, stairs will appear. Go down the stairs.
Go down the ladder and approach the Thwomp to make it fall, then go past it while it moves upward. In the next room, do the same thing. When you are to the left of the second Thwomp, equip the Roc's Feather and quickly jump to the small ladder, climb up, and jump on top of the Thwomp. Then jump from the top of the Thwomp to the upper-left platform. Then go up the ladder to the next room.
Go south and open the chest to get the Nightmare's Key.
Get Another 50 Rupees
There is another optional 50 rupees chest. Go north and jump down from the ledge, then go east twice. Swim north and open the chest to get 50 rupees. Then go south again.
Reach the Nightmare
In the room northeast of the chest where you got the Nightmare's Key, swim to the switch and step on it to open the north door. Go through.
Jump to get the flying hearts, then use your key to unlock the key block. If you didn't open the chest to the right, you can go south, then swim north and open the chest. After that, you can push the lower block to the left to reach the stairs.
Go down the stairs.
Go down into the water and swim east. You can use your sword while in the water to kill the Cheep-Cheeps. In the easternmost room, go up the ladder.
Go west to the next room.
Push the block to the left, then kill any red Zols that get in your way.
Go north through the nightmare's door, then go down the stairs in the next room. Go down the ladders and swim down to reach the boss.
Angler Fish will swim upward and downward. Small Angler Fishes will slowly swim across the room as well. You can kill them with your sword. Sometimes Angler Fish will charge forward and hit the wall, making ceiling tiles fall. Swim out of the way of the ceiling tiles to avoid getting hurt by them. Hit the light on Angler Fish's head to hurt it. Swing your sword rapidly to do a lot of damage at once.
Get the Musical Instrument
After Angler Fish dies, take the Heart Container that it drops, then go back up to the previous room, and climb up the ladder. Go north from that room.
Walk up to the instrument to get the Surf Harp.
Afterward, you get a hint to go to Martha's Bay.