Inside Jabu-Jabu's Belly

Inside Jabu-Jabu's Belly is the third dungeon in Ocarina of Time and is set inside Lord Jabu-Jabu's body. To enter, Link must drop a fish from his bottle in front of Jabu-Jabu, causing him to open his mouth and suck both the fish and Link inside. While inside, Link must fight through a variety of parasitic enemies, including an especially large tentacle-like organism with electrified appendages in the shape of unstable pillars and jellyfish-like creatures called Bari, which become much easier to kill after finding the Boomerang, the weapon found inside the dungeon. Shortly thereafter, Link locates Princess Ruto, inside what appears to be the stomach, who commands him to help her recover the final Spiritual Stone, the Zora's Sapphire. Link must carry Ruto throughout many rooms to help her find it. He will enter a large room with the Zora's Sapphire in the middle.

After taking the Zora's Sapphire, Ruto is captured, and Link is ambushed by a Big Octo that must be defeated in order to continue. After defeating it, Link continues until he eventually locates the creature causing Jabu-Jabu's discomfort: a parasite called Barinade, the dungeon's boss. Link eventually defeats Barinade, rescues Princess Ruto, and along with her, the Zora's Sapphire.

Master Quest
This is a pretty weird dungeon in the Master Quest version of the game. The main change is that there are now cow heads popping out of the walls which act as switches. Even a regular cow is found inside a room. Bombchus are obtained in the dungeon, and are required to solve a puzzle. There are also spider webs inside. Like-likes and Lizalfos now appear, which are found in connecting rooms. The Parasitic Tentacles are found in different locations. The Business Scrubs are missing, and Tailpasarans are nearly absent. The map, compass and Boomerang are obtained really early, and the locations of the latter have been switched.

Note
In the room where Link fights Big Octo, if he attempts to enter the room without Ruto, he cannot get to the Zora Sapphire on his own. He must throw Ruto into the circle to get it.

Minor Enemies and Traps
Shaboms Biri Octoroks Business Scrubs Stingers Bari Tailpasaran Parasitic Tentacles

Trivia

 * A similar dungeon called Jabu-Jabu's Belly can be found in Oracle of Ages.