Unlock the Salvage Arm
In order to obtain the salvage arm, you must first make it through the Temple of the Ocean King on Mercay Island. The Temple of the Ocean King is the third temple in gameplay, following Temple of Fire and the Temple of Wind. After all three temples have been completed, sail your ship to Cannon Island, where more gamplay has now been unlocked.
Obtain the Salvage Arm
Walk into Eddo's house on Cannon Island. Talk to Eddo and he will offer to sell you the salvage arm, but will ask you to shout first. You must now shout (or blow) as loudly as you can into the microphone of your DS. The louder the noise you make, the better the price Eddo will give you for the salvage arm. A very loud shout will result in a price for the salvage arm of 200 rupees, while a very quiet one will end up costing you closer to 1000 rupees.
Find the Sun Key
The salvage arm can be used to haul up treasure at any of the red X's on your sea chart. Sail your ship until it is directly over the X on the sea chart and then select "Salvage" from the menu. Use the sliding bar at the bottom of the screen to guide the hook of the salvage arm to the bottom of the sea, avoiding contact with any obstacles, including rock ledges and floating bombs. In order to find the Sun Key, sail your ship until it is directly above the mysterious green circle marked on your sea chart. Use the salvage arm at this location to haul up a treasure chest containing the Sun Key.
Molida Island
Sail to Molida Island, being careful to avoid explosive barrels near the port. Follow the stone path past a house, the mailbox and the back of a second house. Stop at the first tree just behind the second house. Use your shovel to dig a hole directly in front of this tree. You will reveal a hole in the ground. Jump into this hole.
Down The Hole
Follow the path to the left and then up until you reach the door, using your boomerang to help defeat enemies you encounter. Use the Sun Key to open this door. Follow the path beyond this door to the stairs, where you will now have access to the northern half of Molida Island and the Temple of Courage.