Instructions
Enter Episode 6 of "Ricco Harbor." This is the episode that requires you to collect eight red coins to earn a shine.
Run right and speak to the character on the dock. You will then ride a jet-ski squid around the water. Collect all of the red and gold coins here. It's important to grab these coins first because it's easy to run into boats and other obstacles in the harbor, which cause you to die on impact.
Head into the sewers once you have safely collected the coins over the water. The majority of coins in this level are found here. To enter the sewer, jump and use the ground pound maneuver on the blue and gold sewer cover on the dock.
Inspect every passage in the sewer, and also jump out at every place you see a cover to reach the tops of buildings in the harbor to find more coins.
Head back outside the sewer and grab the handful of coins you find around the outside of the buildings there.
Jump on the boat docked at the port to access the scaffolding above the water. Make your way across the obstacles here to collect the last of the 100 coins.