The Legend of Zelda: Ocarina of Time Tips

Extra Key in Water Temple
1) Go into the Water Temple, you must have not yet opened the locked door at the bottom of the 'tower' in the main room.
2) Enter the Temple, find the room with Princess Ruto, float up and play Zelda's Lullaby next to the Triforce symbol to lower the water level.
3) Now play the Serenade of Water and choose to warp to Lake Hylia.
4) Re-enter the Water Temple.
5) Climb out of the water in the entrance and look at the main 'tower' in front of you, there are ledges encircling the tower. Directly in front of you there is a gap where a block floats up to when you raise the water level, don't jump there. Instead, take a running jump at the ledge to the left of the of the gap, you'll miss the highest one but will manage to land on the next one down.
6) Located on this ledge is a barred door and a lit and unlit torch, light the unlit torch with your bow or Din's Fire to enter the 'tower'.

You are now inside the 'tower' without having used a key to open the locked door at the bottom of the tower, so you have an extra key (or will have), this means you can skip getting a key somewhere or have an extra one. Opening the door from behind uses a key like normal.