You go to the room above it. There's a part in the water in that room to blow up, but not in the room with the boss key.
I thinkbits 5-6-7
If you're talking about in twilight princess, there is a lake bed temple. Complete all the steps to the temple. You should eventually end up in an underwater maze. At the end, there is a room with three spider web covered holes in the floor. Cut the one in the middle, fall through, and the boss' key should be beneath you.
There is none...
The big key
well the toad mini boss is were you use the ice arrows to make a plat form in the room were you got your first key and melt the ice with fire arrows that's it
7th floor the room with the armos
In the Forest Temple the third monkey is located in chamber eight lair of the big baba and the monkey is behind a locked door. Defeat the big baba and the key falls from it. Use the key to unlock the door to the monkey.
First you shoot the blinking eyes then go to the key door defeat monsters then step on the swichtes
(Note that you have to have got 7 badges and you have done the events at the lakes, Lake Acurity, Lake Valor and Lake Verity.) Go in front of the Galactic HQ in Veilstone city (the big one) the should be a galactic grunt talk to him and he'll run off where he was is the galactic key. Hope this helps ;-)
there is no key for valor lake front
There is no switch except for two levers you pull down on either side of the temple, all the way to the left and right, and all the way up. Once you have pulled both of them and you have the boss key, you can face the boss.
The room where the key is is directly above the "main entry chamber" (the big one with 4 doors right near the beginning). You have to go all the way through the temple, as the bridges are out. You'll eventually reach a point where you need to double-clawshot your way across the missing bridge using those desert-helicopter-basket things. The key is in a tiny room with a big fan in the floor (same fan that's in the ceiling of the main entry room). You can stop the fan with a switch and jump back down through the hole to get back to the main room of the temple, and onward to the boss.