With the Northwestern Sea Chart in hand, you can now sail to the Northwestern Sea.
Try to Sail North
If you try to go north from the east side of the map, a tornado will throw you back. Go west to the gap in the rocks, but be sure to use your cannon to bomb the cracked rocks that are blocking the way. If you don't have the cannon, go to Cannon Island to get it.
From there, you will see the Ghost Ship. Try following it north to the Northwestern Sea. Eventually the fog will send you back. You have to go back to the Southwestern Sea and visit Molida Island for clues. Be careful not to hit the bomb barrels floating near the Molida Island dock. You can hit them with your cannon to get rid of them.
When you talk to the people here, they tell you to talk to Romanos, who might know how to get past the fog. He lives in the easternmost house in the southern part of the island. Talk to him and say Yes! when he asks if you are going your own way. He refuses to say more, so leave. But then he asks you to come back, so go back and talk to him. He says that his father knew a specific route for getting past the fog. He says to go into the cave behind the house for more clues.
Go into the cave behind the house. Go west in the cave, then up the stairs. Run to the right to jump across the gaps. Go north from there, and a Zora Warrior will come up from the water and attack. You have to dodge when he attacks, then hit him before he can raise his shield.
After you defeat him, go north. Follow the straightforward path to the next area of the cave. Then use a bomb flower on the cracked wall. Go into the cave and check on the book. It says that Romanos's father has another hideaway, and it can be found if you draw lines between the stone tablets that he put around the island and go to the place where those lines intersect.
Open the chest in this room to get the Shovel.
Leave the room and use a bomb flower to blow up the cracked rocks southwest of Romanos's father's hideaway. Go south from there to return to the southern part of the island, and make a note on your map of the positions of the two stone tablets there.
Then go north to go back into the cave, and go northwest to reach the stairs. Go up the stairs to reach the middle part of the island. Make a note of the stone tablet that you find here.
Go southwest from that stone tablet to find a chest.
Then go east to reach a ledge in the southern part of the island. You will find the fourth stone tablet here. Make a note on your map of the location of this tablet, then draw a line between the stone tablets that you marked on your map.
Go downward to fall off of the ledge, then go to the place on your map where the lines intersect. It is north of the woman in the yellow shirt who is standing next to one of the tablets. Dig in front of the palm tree that is north of that woman. A hole will appear, so jump in.
Read the book here, then open the chest to get a treasure map. Then check the map on the wall to see the secret path through the fog. Draw the path on your sea chart.
Go west and go through the path. Ciela tells you to try the door, but it won't open. Draw the symbol from the door on your Southwestern Sea Chart near this island.
Go east, and the path will lead you back to the part of the cave where you went earlier. Go southeast to leave the cave.
Before you go, dig in front of the palm tree to the right of Romanos's house to get a big green Rupee.
Then talk to Linebeck to set sail.
You can visit the Traveler's Ship near Molida Island before you go. Inside, kill the Miniblins, then talk to the guy on the ground until he gets up. He is Nyave. He gives you a treasure for saving him, and tells you about his brother in the north.
You can also visit the hidden island in the middle of the triangle of rocks that is south of Molida Island. When you get close to it, Ciela will see it and add it to your sea chart, so you can set a course to dock there.
On the island, go east, then cross the small bridge, then go east to find a treasure chest.
You can go north into the cave and approach the fountain, but if you do not yet have enough Spirit Gems, you will not be able to do anything here yet.
Escape the Fog
Talk to Linebeck to set sail, then go north from here to return to the foggy Northwestern Sea. Draw your route according to Romanos's father's map that you found on Molida Island. Be sure to keep an eye out for enemies and bomb barrels as you go.