Finding the Magnifying Lens
From the entrance of Catfish's Maw swim over to right side, walk up the stairs and follow the path down to the south, use the Hookshot to pull yourself across the gap and continue up the path until you find a Mermaid Statue. Place the Scale you received earlier into it and a staircase will be revealed.
Note: The Magnifying Lens will allow you to see things in the game that weren't visable previously.
Head down and pick up the Magnifying Lens, exit the underground area and make your way into the building on the far right of Animal Village, speak with the Zora and he'll give you a Secret Seashell. He'll then inform you that there is someone else like him in Tornbo Shores.
Exit the house and follow the path around the outside of then fence to the northeast and place a Bomb against the crumbling wall, inside you'll want to push the stone off the side to get a little further forward and then hop over the gap, now follow the path up and equip the Bow and Arrow and the Bomb, activate the two items at the same time to fire a Bomb across to the left causing the stone to break.
Obtaining the Frog's Song of Soul
Use the Hookshot to pull yourself across and collect the Piece of Heart that's sitting near the top of the stairs, now hop down to the right and place a Bomb against the crumbling wall and exit the cave from where you entered. Take a left and keep following the path to a dead end and pickup another Piece of Heart.
Make your way over to Martha's Bay and head onto the beach, go south as soon your feet touch the sand and dig around near the ocean to find a Secret Seashell, follow the path back up and around until you come across another crumbling wall then place a Bomb against it, head inside and speak with Goriya who will propose a trade for a Boomerang.
Note: Here you'll want to trade your Shovel for a short period of time, as you'll be able to trade it back at any point.
Use your Ocarina and teleport to Manbo's Pond and make your way over to the cave where the three stones have been blocking your path, head inside and use the Hookshot to pull yourself over to the Chest that has 50 Rupees inside, then grab the Piece of Heart to the left.
Exit the cave and make your way into Ukuku Prairie, then head south and use the Hookshot to pull yourself over the holes to the right, continue south a little more to take on a Puzzle. In order to complete this puzzle you'll need to interact with the sign post at the top and then follow the path it sets out for you.
Once you've succesfully interacted with the sign posts in the correct order, a staircase will be revealed, proceed down to speak with Mamu and pay 300 Rupees a new song, Frog's Song of Soul.
Locating the Face Key
Head back up to the surface and teleport over to the Animal Village, then exit the village and head north until you reach a right turn, continue all the way right until you see an opportunity to go down into the Ancient Ruins. Make your way over to the center of the ruins and take out the guards by hitting them with your Boomerang, after they've been defeated you'll see a staircase, go down and open the Chest for a Secret Seashell.
Exit and head all the way left and enter the building ahead, inside you'll need to battle a large guard, in order to defeat him you'll need to circle the small arena whilst throwing your Boomerang at him until the Shield breaks, then you can kill him by using your Sword. Pick up the Face Key from the floor and enter the room ahead, light the two candles with your Magic Powder and inspect the wall to find out some backstory on the Island.
Exit the building and head out of the Ancient Ruins, make your way onto the small platform in the water and kill the two guards to reveal a staircase, go down and exit out of the otherside and place the Face Key into the statue to open the Face Shrine.