It's at the bottom of Lake Hylia. You should have gotten the Zora Armor by now. From there, put it on and just go the place marked on your map.
in the main room
you need to go to the lakebed temple in lake hylia
kill the lakebed temple boss or collect enough heart pieces
his in a room on the ceiling
In the sea temple.
If you want to find the BOSS for the Lakebed Temple on Twilight Princess, you have to get the BIG KEY and then go to the room with the giant swiveling staircase, and just jump into the water. The BOSS DOOR is level with the water surface under the swiveling staircase. This BOSS is the coolest one so far. -Caveman1593
In the first temple
Death Mountain
Beat the first three levels. Forest Temple, Goron Mines, and Lakebed Temple.
For the first part, use the clawshoot to pull Morpheel's eye out. For the second part, chase after him and use the clawshhot to pull yourself to his eye and attack it.
It's on the map.
beat the boss