Follow the one and only path available, past the room with the Song Plant in the glass case, into a large chamber.
Swim left to find a Save Crystal. Head up to find a gear, spin [Hotkey R] next to it for a few seconds, to increase the water level.
Proceed to the right room with another Song Plant, hug the right wall to reveal a secret passage. You can skip it now and wait untill you've acquired the Sun Form, but to think all the hassle you have to go through and all the backtracks, you'll agree with me and rather do this even in the dark :D
Within is the Sun Key treasure,
and a Song Plant containing the Royal Soup recipe.
Up to the top, bind the blue crystal then pull it over the passageway. Swim back to the gear in the main room and spin again to lowering the water level.
In Beast Form, jump over to the right room to find the blue crystal has been lowered through the passageway. Bind and drag it against the left wall. Now spin the gear again.
The water will raise the crystal into the middle room, so swim up there. Bind and drag it again. This time over the vertical passageway in the middle of the room.
Lower the water level, bind and drag the crystal to the passageway of the left room. Spin the gear for the last time,
Bind and drag the crystal left into the sunlight to charge it, apparently it gets too heavy for the holder, which tips it out.
With the crystal in tow, head downwards into the dark room. The blue crystal is your only light source here, now find a door on the left wall. The sun door will silently shows musical notes, repeat the notes to unlock the door.
Follow the path, at the very top off the room, you'll find an encased Song Plant.
Swim left until you reach at a room with a pipe opening on the left wall.
Enter the pipe to come across a secret room containing the Golden Gear treasure.
Continue on into the next room. Spin the gear to raise the water level.
Swim through the right path to find another large room. There's a Save Crystal to the left of this room, use it. Head back to the previous room if you want to find Naija's lost memory, or take the right path to fight the boss.
[Optional] Head back into the gear room, use your wall jumping skill in the upper left shaft.
[Optional] In the next room, jump to the bottom right corner for a Song Plant containing Leadership Roll.
[Optional] Jump from gear to gear to get to the upper right legde.
[Optional] Proceed to the next room, blast the case, grab the Turtle Soup recipe.
[Optional] Once again, you need to jump from gear to gear. Just keep trying and you'll eventually make it to the top. If you fall through the gap in the middle of the room, just come back again the same way you came.
[Optional] Swim up through the bubble.
[Optional] Wall jump your way up the glass pipe into the huge bubble at the top to unlock another of Naija's lost memories. Afterwards, swim back down into the temple and through the gap.
Hit the save point to the left, then proceed right to enter a cave which full of spikeyeggs. You need to wait for the right timing as the water level fluctuates consistently.
To defeat the Sunworm, just shoot him, avoid his attack, grip the wall to avoid getting sucked in. Make sure to bring a plentiful supply of healing items.
Acquiring the Sun Form [Hotkey 5].
Once you've defeated the boss, the water level is fixed quite high. Climb the two long tunnels for a Special Cake and a Special Bulb.
Leap out of the shattered window to get back to The Veil (map II).