From the start, walk down the ramp and into the water. Pass by the Rammerhead to reach Green Star #1, then head back towards the ramp, making sure you stay in the water. Hook around the pipe to the backside of the main wall, where you can find a Super Mushroom and some clear pipes. Ride the pipe up to the right where Green Star #2 is located.
Take the pipe once more, but into the other exit this time. Follow the path, dropping onto the wooden planks, then head through the hole on the far end. Make your way to the edge and drop off to the raised level, where you can strut to Green Star #3, then into the pipe. At the top, you’ll want to fall down the hole, onto the P-Switch to bring Green Star #4 right to you. All thats left to do is to walk carefully by the Rammerhead along the wooden planks and up the ramp to Green Star #5.