The Temple of the Ocean King has a sun symbol clue. Use the salvage arm to solve it.
Get the Salvage Arm
Go toward the dock of Mercay Island and the postbox will be wiggling. Approach it to get a letter from Eddo, who has finished the Salvage Arm.
Talk to Linebeck, and set sail. Then go southwest to Cannon Island. Go into the building, and go to the back room and talk to Eddo. He will sell you the Salvage Arm, but you have to shout into the microphone to get it. If you don't want to shout, you can try tapping repeatedly near your DS microphone with the stylus. Eddo's price can range from 200 rupees to 1000 rupees, depending how loud you shouted.
Now you can go to any red Xs that there might be on your sea charts to search for treasure. Just stop at a red X, then tap Menu, then tap Salvage. You can drag the controls left or right to move the arm left or right, and you can drag upward to slow the salvage arm down or drag downward to speed it up. When you find the treasure at the bottom, you have to make the arm grab it, and then you have to pull the arm back up without hitting too many things. You can find many treasure maps around the world.
If the Salvage Arm gets damaged, you have to dock at an island and visit the dock yard building next to the dock. Talk to the guy there and pay to have the Salvage Arm repaired.
When you are ready, go to the green symbol on the Southwestern Sea Chart and use the Salvage Arm there. You will get the Sun Key.
Molida Island
Now sail to Molida Island and try to dock there. A Giant Eye Plant will pop up out of the water. Draw a route that makes you sail back and forth in front of the monster. That way you are less likely to get hit when it spits projectiles at you. Shoot the monster's eye. It will close its eye and spit spiky green projectiles at you. Shoot the projectiles to prevent them from hitting you. Then shoot the monster's eye when it opens again. Repeat this strategy until the monster dies.
After you dock at Molida Island, dig a hole in front of the palm tree at the intersection of the lines that you drew earlier. Fall in and go west. Unlock the Sun Door at the back of the cave. Go through to reach a new area.
In the new area, there is a patch of grass that looks weird. Dig there with the shovel, then fall into the hole that appears. Open the chest to get a Wisdom Gem. Then go east and up the stairs to return to the new area.
Go west. The statue along the path tells you about a treasure hidden near Oshus's house. Open your sea chart and tap Mercay Island to make a mark where the statue said the treasure is.
Go east from there, and you will see a statue head on a hill. Go east and go up a short flight of stairs, and before you continue, use your boomerang to hit the stone head on the hill. Then go north up a small bridge, then go east to find the temple. You can't get in, but the stone tablet next to the door gives you a clue about how to open it. Go east and you will find a stone head. Hit it with your sword to make it shine a beam of light. Then tap the statue to rotate it. Make it rotate so its beam is shining on the temple door.
Go southwest and follow the path. Two Zora Warriors will attack. If you are quick, you can use your boomerang to stun one or both of them. When they are stunned, hit their sword arms (otherwise your sword will hit their shields instead).
After the Zora Warriors die, another statue head appears. Hit it to light it up, then tap it to rotate it. Turn it to point at the temple door.
Go back to the temple. If you have only activated two statues, go southwest from the temple and look for the statue head on the hill. Hit it with your boomerang to activate it.
Now go back to the temple and go inside.