After entering the deity's mouth by offering him a fish shoot the uvula (the yellow thing hanging from the ceiling) to open the door.  Pass through this room and you'll enter a room with pits in the floor.  By one is Ruto.  Talk to her.  After shes falls down the hole follow her.  Talk to her again, and then talk to her again.  Now she will make you carry her!  Head through the door behind you.  Jump in the pool and throw Ruto to the other side of it.  Now step on the switch to get out.  If you ever lose Ruto head back to where you fell in the hole to get her.  pass through these next passages and ride the elevator up. 

This is the room with the big pit that you went through before.  Walk to the room with all the holes in it and pass through it without falling in any.  Move forward some and head right.  Try to avoid being attacked by the electric worms. After defeating all the enemies in the room at the end of this passage you'll get the Boomerang.  Leave the room and go straight until you get to the next room.  Place Ruto on the switch to open the door.  Go in the room and destroy the tentacle with your newly acquired Boomerang.  Take the map and get Ruto.  Head left at the next opportunity.  Beat the tentacle here and get the compass.  now when you get to the intersection with the electric worms head left.  Fight the tenacle here and then fight the tenacle in the only passage you haven't gone to in this area.  After that head back to the room with the holes.  On the far right side there's a pit.  Fall in it. This is the lair of Bigocto.  Throw Ruto on the platform to retrieve the stone.  After she disappears fight Bigocto with your boomerang.  Hit him in the back with it.  He'll be stunned.  Then you should attack him with your sword.  Continue this to defeat him.  TIP: If you run close to the spikes you can go a little faster than Bigocto.  Beware when he changes direction. Once Bigocto's through with ride the elevator up to the next level. After entering the room with big swaying red thing you should hit the m with your boomerang to form platforms.  Hop across them to enter the room with the big pit.  Go to where the new floor is and enter the door there by taking a crate by the wall and placing it on the switch.  In this room there are lots o jellyfish.  Destroy them with your boomerang and then scale the wall.  Use your boomerang to hit the switch blocked by the web.  Now go in the unlocked door.  Good Luck!