the water temple is in Lake Hyrule. you must have the iron boots.(you get those in the blue ice cavern near the area where lord jabu jabu was as a kid) go to the lake and get into the little thing of water hit the switch thing above the underwater door with the hookshot under the island in the middle of the lake. the door should open then just go inside.
Well, it sounds sarcastic but, you complete the temple (duh :D) If you mean how do you complete the temple then try looking at a walk through :) Walkthrough: strategywiki.org/wiki/The_Legend_of_Zelda:_Ocarina_of_Time/Water_Temple
Ahoy, there o/ You HAVE TO go to The Shadow Temple before, to get the Hover Boots and walk through the Wasteland. For you to enter the Spirit Temple you must have past through all the other temples, but the only one you need to complete it's the Water Temple. You can simply get the Special Item inside every temple, make your way till you reach the Bosses' Door and do not defeat the boss (except for the Water Temple), then you can go to the Spirit Temple.
the water temple item is the longshot
water temple
The Water Temple is located at the bottom of Lake Hylia. To get to the water temple, you must first obtain the Iron Boots, and the Hookshot.
The water temple's boss is Morphia, an eye with tentacles made of water.
Sunol Water Temple was created in 1910.
Pulgas Water Temple was created in 1938.
you have to beat the fire temple.
shadow temple :)
In the Lakebed temple/ water temple.
Water Temple.