Before you go to the Northwestern Sea, you should stop at Cannon Island to get a new part for your ship.
Set Sail
After getting the sea chart in the Temple of the Ocean King, go back to the dock. Someone on the dock will tell you to visit Eddo on the island to the south to get a cannon for your ship.
Talk to Linebeck. He will say that the map is too dusty to read, so blow into the microphone until the dust goes away. (Be careful not to hyperventilate.)
Before you go to the Northwestern Sea, you should visit Cannon Island to the south.
On Cannon Island
Go into the building on Cannon Island and talk to Fuzo, who will open the gate west of the dock.
Go through that gate and follow the path until you see beehives. Go east from there to reach a clearing. Open the treasure chest to get a treasure map.
Then go southwest and into the cave.
Go east and tap a bomb flower to pick it up. Then put it next to the blocks on the right, and walk away to a safe distance. When the bomb explodes, go east through where the blocks were.
Get another bomb flower and go to the southeast corner and throw the bomb at the crack in the wall. Go north and open the chest to get a Power Gem.
Get another bomb flower and throw it over the fence at the block next to the metal block. Then push the metal block to the right until you can go north. Pull the next metal block downward until you can push it to the left. Then go north and pull the next block to the right, then up. Then go back and pick the bomb flower and take it to the northwest area, and throw it at the block before it explodes. Then go west and go up the stairs.
Outside, use the bomb flowers to blow up the blocks to the south and the other blocks to the right of the fence. Then go southwest and open the chest to get a red rupee. Go east from there and blow up more blocks. Then go north and get a bomb flower, then jump down the north ledge and go east. Throw the bomb flower at the blocks there.
After those blocks are gone, go north from the area with the bomb flowers, then follow the path east to where you blew up the blocks. Keep going east and you will reach another entrance to Eddo's Garage.
Talk to Eddo, and pay 50 rupees for the cannon. Then you can go west and exit through the door there to return to the first room of Eddo's Garage. Then go outside.
Check on the postbox if it is wiggling.
Then talk to Linebeck when you are ready to leave.