Instructions
Purchase the Marlin Rod from the merchant at Sei-An City for 10,000 Yen.
Travel to North Ryoshima Coast and swim to the western island to talk to the fisherman on the dock. He will take the Marlin Rod.
Press the corresponding button to bring up your Celestial Brush and paint a line from the Marlin Rod to a fish to help the the fisherman catch it. Just before the fisherman catches the fish, a button prompt will flash on screen. Quickly press the correct button to pull up the fish and then bring up the Celestial Brush to draw a line through the fish to knock it out. Big fish require additional lines. You must help him catch three fish, the last one being a marlin, to progress.
Swim back to the North Ryoshima shore and walk up to the restaurant. Talk to the fisherman and you will be given a prompt to use the Celestial Brush.
Draw three horizontal lines in the same design as the picture in the background using the Celestial Brush. Amaterasu can now use the Whirlwind attack by painting the same design with the Celestia Brush during battle.