This page offers a walkthrough for the Underwater Cosmic Mario Race Power Star in Sea Slide Galaxy. This galaxy is found in the Engine dome.
Challenged by Cosmic Mario
Landing on the starting island, Cosmic Mario will appear and challenge you to a race. You start the race by running into the water and begin swimming. This will be harder than racing penguins, as Cosmic Mario will use ride a shell to swim through the water.
Be sure to grab a shell of your own to keep up. You will need to use the rings to boost past Cosmic Mario. Red Shells wont be able to hit Cosmic Mario. You wont need to go round the complete course this time, as you will see arrows pointing Mario to go back on land with the tower. The Power Star will be set behind the tower.
Head out the water, onto land and up the steps. When you’re on land you can throw the shell and make your way to the Power Star that should be next to the tower. Be sure not to hesitate as Cosmic Mario will be close behind.