Important note - if you crash into an item, you end the game. So be careful!

You stomp the red thing to start the timer. Don't stomp right away - you can just explore first to practice, you don't get red coins yet. When you know the course and are ready, do the red stomp and start.

It's pretty easy, all 8 are right in the water. The tricky part is you have to jump up on dock to end the game, if you smash into the dock you lose. You have lots of time.

Note that the colors do matter!!

green seems slower, hard to jump though
yellow is middle Do one or two with this to gear up and then switch to green - it'll seem easy
purple is fastest - easy to get blue coin at jump

first 4 in straight line, left for 5, 6th at far end, then turn back. 7 is under orange structure and 8 is between 2 boats back by start

Super Mario Sunshine Walkthrough