Do you already have all the keys needed to get there, or have you just started navigating through the temple? Oh well, a little more information is better than a little less..
To get the three keys needed:
Note: You enter the temple from the South on floor 3F, I will use this for reference.
1) Sink, with the Iron Boots, to floor 1F and go through the East corridor. You will meet Princess Ruto. Follow her up to the surface, then play Zelda's Lullaby in front of the blue stone tablet (enter the room and defeat the Shellblades for the Map). Then fall to the lowest floor and light the torches with arrows or Din's Fire. Inside the room on the lowest level, you'll find the first key.
2) Go to the West corridor on floor 1F and push the block in the wall. Follow it and find a room with a fountain you must activate with a crystal switch, and jump on top of to get over. You will find a room with a square pool filled with strong currents. Dive to the bottom, hit the switch in the dragon statues mouth, remove the boots and float through the gate that opened. You'll find the second key.
3) Unlock the central turret in the temple and go inside. Find the second blue stone tablet and play the lullaby. When the water rises, dive down underneath where the wooden platform was located, beneath the door you came in through. Sink to the bottom, find the switch and hit it, defeat the enemies and go through the gate that opens. Pick up the third key.
4) Go back to where Ruto was (East corridor floor 1F) when the water is at mid-level. Find the cracked wall on floor 2F and bomb it. Get the fourth key.
Now let's find Dark Link:
1) Go to the West door at floor 2F and unlock it (with the water at mid-level), stand on the fountain and hit the crystal switch. Rise to floor 1F and find the third blue stone tablet. Raise the water to the highest level.
2) Go through the West door at floor 3F and unlock it. Use the hookshot on the dredger of floating platforms in the waterfall and work your way up to the top. unlock the door.
3) Raise and lower the water in the room with the crystal switch. The dragon statues will lower and raise with the water. Repeat and work your way through the room and climb on top of the last statues when its lowered. Raise the water to raise the statue, defeat the enemies and use the hookshot to reach the last door.