Locating the remaining Pieces of Hearts & Secret Seashells
Although you've awoken the Wind Fish and technically completed the game, you can still load up your save and continue exploring Koholint Island to get the remaining Pieces of Hearts and Secret Seashells.
Fishing Related Collectibles
To begin, you'll want to travel over to Mabe Village and speak with the Fisherman there as there are two Secret Seashell's and two Pieces of Hearts available to get from fishing. After the Fisherman is done showing you the ropes you'll want to go in and catch any fish for a Piece of Heart, then you'll want to catch the Lunker which is the large Green fish at the bottom of the pond, getting him will rewards you with another Piece of Heart.
Now you'll need to catch two rare types of fish, Cheep Cheep and Blooper, this fish can only be caught when the pond has sparkly spots on top of it. In order to capture the Blooper you'll need to wait for the fish to stop trying to swim away from you then attempt to reel it in, the Cheep Cheep will require you to wait for it to begin swimming after landing back in the water. Catching these fish will bag you two Secret Seashells.
Rapid Related Collectibles
Make your way back down to the where the Face Shrine is located, but keep going right and proceed into the cave and Hookshot across to find a Piece of Heart, head up the stairs and speak with the Raft Shop owner and chose to do the Rapid Race.
Tip: To catch the Secret Seashell and Piece of Heart within the rapid you'll need to equip the Hookshot and use it pull yourself against the water.
Once you've began your rapid adventure you'll want to travel all the way to the left and Hookshot down the left side to find a Piece of Heart, then pull yourself across to the center to find a Secret Seashell. Now complete the Rapid in under 40 seconds to be rewarded a Piece of Heart.
After getting all you can from Fishing and the Rapids it's time to travel into the Mysterious Forest and go back to the cave with the three stones blocking the entrance, from the entrance you've go south a little and dig up a Secret Seashell that's hidding in the middle of a circle of blue flowers. Now make your way towards the northwest exit and dig a hole below the tree to find another Secret Seashell.
Go up and hop past the holes beside the Telephone Booth and dig above the booth to find a Secret Seashell, now head over to Tabahl Wasteland and hop over some holes to find a Piece of Heart then go down and right and dig another hole to find a Secret Seashell.
Make your way over to the location where Tarin got chased by bee's earlier in the game and go south to where you destroyed the large skull, dig a hole to find a Secret Seashell then go into the field behind Richard's Mansion and head to the back right corner to find a Piece of Heart.
Exit the mansion and go to the south of the Seashell Mansion where you'll find some stairs to the right of some trees, proceed down and dive into the small patch of deep water to find a Piece of Heart. Exit the cave and head into the Animal Village to inform Christine that you've successfully delivered her letter and she'll reward you with a Secret Seashell, now play the Balled of the Wind Fish on your ocarina at the entrance of the desert for another Secret Seashell then continue to the top right of the desert and dig a hole between some cactus below the Owl statue for a Secret Seashell.
Leave the Animal Village through the entrance you came in and head north and right, remove the stone in the top right corner to find a Secret Seashell, now dive into the water and swim behind the Kamlet Castle until you stumble across a waterfall, dive down into it to find a Secret Seashell.
Go back to the right and climb up the ladder then throw a Bomb into the hole beside the teleporter to get a Secret Seashell to fly out.