Northern Fairy Island is a small, heart-shaped bit of land with a large conch shell sticking up from its middle. One of the Great Fairies dwells here and upgrades your maximum Rupee-carrying capacity.
Blue ChuChu
Defeat the Blue ChuChu near the eastern palm tree for a Blue Chu Jelly.
Treasure Chart #22 (via Submarine)
Treasure Chart #22 is in a submarine in this sector. Old Man Ho-Ho is looking at the submarine from the island, using his telescope. Sail to the submarine, then climb onto it and enter the doorway leading inside.
This is the only submarine in which you don’t have to fight any enemies to get the treasure. Five Moblins patrol the lower area of the sub, but you can use the ropes of the hanging lanterns to reach the chest on the other side for Treasure Chart #22.
Rupee Wallet Upgrade (via Great Fairy #2)
Bomb the large boulder to open the way into the conch shell. Drop down the hole inside to enter a Fairy Fountain where you find a Great Fairy. The Great Fairy upgrades the amount of Rupees you can carry and invites you to return any time if you need to replenish your Life Gauge.
Silver Rupee (via Treasure Chart #24)
Find and open Treasure Chart #24 (at Windfall Island [D2]) to pull a chest from the Great Sea near Northern Fairy Island. The chest contains a Silver Rupee.