Once you finish Key Cavern the Owl says you have to make your way to Yarna Desert and the monsters found there will show you the way forward to the Angler Key and ultimately Level 4 Anglers Tunnel.
The Lazy Walrus
Make your way east to Animal Village and have a chat to some of the locals there. The bunny found in the first house tells you that Yarna Desert is close, but you might not be able to get access due to a lazy walrus being in the way. Talking to the bunny outside the house he says it’d be nice if Marin popped by and sang everyone a song.
Exit the house and go out of Animal Village to the west and then South, where you are met with the Walrus asleep and in your way blocking access to Yarna Desert.
Go and get Marin
The Walrus is asleep and the only thing that’s going to wake him up is if you go and get Marin to sing him a song and wake him up. Go back to Mabe Village and have a chat with the little boy who’s standing near the weather vein where Marin’s normally singing. However, Marin is nowhere to be found.
Go back to Marin’s house and there’s a note left on the table from Marin saying she’s at the beach. Exit Marin’s house and go back to the beach near to where you found your sword at the start of the game. Marin’s there and she’s got a little story for you.
Marin explains she wants to see beyond the seas of Koholint Island and when Link arrived it gave her hope that something else was out there. She says she wants to travel far away and soar like a seagull singing to everyone she meets. Link explains the Walrus in Animal Village needs her and now Marin will follow you wherever you go.
Wake up the Walrus with Marin
Now go back to Animal Village and find the Walrus sleeping there at the entrance of Yarna Desert. Marin will sing him a song and he’ll wake up and jump into the sea. Once he’s out of the way, Marin will head back to the village and this will allow you to get through to Yarna Desert.
Get the Angler Key
Once you make your way around the maze of Yarna Desert there’s a big worm enemy in a pit of quicksand. Try and avoid falling through the quicksand by staying out of the middle and hitting the worm with your sword.
Once you have beaten him he’ll reveal the Angler Key!