In Ocarina of Time, the boss of the Water Temple is Morpha.
The Water Temple is the sixth dungeon in the adult portion of the 1998 Nintendo 64 game. It lies beneath Lake Hylia, whose waters have been affected by Ganondorf's actions. Link must repeatedly raise and lower the temple's water level to reach new rooms, using the Iron Boots, Zora Tunic, and Hookshot along the way.
Morpha is a large water amoeba that attacks by forming a fluid tentacle and hurling its central nucleus around the arena. Link must pull the nucleus out of the water with the Hookshot and strike it before it returns. This battle is distinct from the Dark Link encounter earlier in the temple, which is a mini-boss-style fight rather than the dungeon's final boss.
The other options belong to different Zelda battles: Twinrova appears in Ocarina of Time's Spirit Temple, Bongo Bongo guards the Shadow Temple, and Shadow Link is an illusionary opponent in the Water Temple.