Jump up onto the platform once Bigocto's dead and you'll be lifted up to a new room.  Go down the only passageway and door and you'll end up in a room with weird red blocks that wobble.  Smack them one with the boomerang and they'll solidify for a while, allowing you to use them as stepping stones to make your way to the door on the west side of the room.  In the next room jump across the gap to the platform and it will lower you to the alcove with the switch you saw at the beginning. Go pick up one of the crates and place it on the switch.  Go through the door that opens.  Kill all the jellyfish here and climb up the vines to the high platform.  Now you should see the white thing hanging from the ceiling, but there's something in the way that keeps you from hitting it.  You can't destroy the thing in the way so don't bother.  Instead, Z target the white thing and then move as far to the side of the platform as you can and you should be able to chuck the boomerang AROUND the barrier to hit the white thing and open the door to the boss.  Climb down and go through the door to face Barinade.

Jabu-Jabu's Belly Boss: Berinade

This boss is tough.  Use Z targeting and your boomerang and KEEP MOVING around him at all times to avoid being zapped.  First, destroy the tentacles.  Then, destroy some of the giant jellyfish he has protecting him to get an opening.  Then attack him directly.  When the boomerang hits and his body flashes blue go after him with the sword.  Just keep going after him and his protecting jellyfish with your boomerang and Z targeting, moving around him at all times, and slashing at him with your sword when you stun him with the boomerang, until he's dead.  Remember don't go after him every time you hit him with your boomerang...only when he's still exposed.  Other times he sinks and when he does that, attack the jellyfish with your boomerang instead of trying for the sword.  Once he's dead get the heart container and go into the blue light to meet Ruto.