This one is pretty hard if you don't do it correctly, and you'll either be able to do it right away or it'll take you forever.
Right at the beginning, talk to the large Penguin, where he'll ask if you're a surfer. Select "yes", and he'll tell you how to surf with Manta Rays. First point to the screen and keep it still for a second or so. Next, twist it left, then right. He'll also explain that you can jump by pressing the A button. He doesn't say so, but you're also able to jump by flicking the Wii remote upwards. Next, you'll go across a large water area where you can practice for awhile, though you'll then have to really start the race.
You have to beat this galaxy in under 1:30.00, so do it quickly! It helps to slow down when taking the turns because there is plenty of time to go through the course.