Water Temple (Ocarina of Time)

Deep under the vast Lake Hylia lies an ancient temple known as the Water Temple. Link must use the Iron Boots and the Hookshot to access its entrance by shooting the target above the gate. Various parts of the temple require Link to go underwater for substantial amounts of time, thus necessitating the Zora Tunic (though the Water Temple can be beaten without it by an experienced and resourceful player). Its enigmatic structure consists of various levels and requires the use of playing Zelda's Lullaby at each of the special markings on the walls. Link will need to use all of his intellectual skills and cunning survival techniques to truly conquer this temple. Link will also come to confront his very own shadow-self in the Water Temple. After defeating Dark Link Link will find the Longshot. Once he acquires the Longshot, he can get the boss key and enter the lair of the aquatic boss, Morpha.

This is often seen as the hardest temple that Link will come across in the game and possibly the hardest in the entire Zelda series.

A rough MIDI of the mystical music in the Water Temple can be found here: Water Temple theme

Minor Enemies and Traps
Blue Tektites Shell Blades Keese Stingers Spikes Like Likes Sink holes Whirlpools Blade Traps

Trivia

 * Due to the frustrating swimming mechanics and the incredible length, many regard the water temple as the most annoying dungeon in Ocarina of Time, and some the most difficult Zelda dungeon yet. Compunding this perceived difficulty is the need to pause the game in order to equip or unequip the Iron Boots, adding a lot of extra time to the dungeon. Ironically, the Water temple is actually easier on Master Quest. The infamous "serpent-shaped" room now has Hookshot targets over the wall, making it easy to simply bunny-hop through the vortex-filled gauntlet.
 * It is very likely that the Water Temple and the Lakebed Temple of Twilight Princess are the one and same.