You need to make sure you know the Frog's Song of Soul, if not, figure out how to get it here. Head to Mabe Village. Rememeber that Rooster Statue where Marin used to sing? Push it aside and go in. Play the Frog's Song at the bone pile to resurect the rooster. Now for a REALLY long trip...

Go to the place where you had to jump off to get to Angler's Tunnel. Instead of jumping, go right in the water and upt he first set of stairs. Go in the nex first cave you see. Use the Rooster to fly you to the Eagle Key.

Go back to the river and swim to the far right of the river. Go up the last set of steps you see. Follow through the cave and emerge from the other side. WHATEVER you do, do NOT fall in that hole. Trust me. Instead go up the stairs and in the cave. Follow through the cave, the next cave, and down the stairs. Jump over the pit and go up the stairs. Follow through the cave, emerge and go into the next cave. Or, if you're low on health, then bomb the cracked wall and get healed by the Big Fairy. Anyways, in that latest cave go up the stairs, and finally make your way through and emerge. Kill the two Tektites and go left. Go put your key in the hole and open up the Eagle's Tower.