At the start of the level, you'll meet Plessie, a sea monster. You must steer Plessie to the goal and press forward or backward to speed up or slow down. Press (A) or (B) to jump. Once out of the gate, you'll find many fish in the water. Jump and land on them to bounce. The first Green Star is over some fish in front of a ramp. Bounce off the fish to get it. It can be a little difficult to line up Plessie to get Green Stars and other objects. The path splits ahead and you can go through a tunnel, over the bridge for Red Coins, or under the bridge for coin rings. Further along, you can either bounce up the steps for a 1-up or bounce off the fish for coin rings.
Ahead you'll see a waterfall. The path splits here. If you do not go through the waterfall, you'll go through a narrow path with fish and the stamp. If you do go through the waterfall, you'll find the second Green Star and a Starman. The paths rejoin on a downward slope with fish and boost pads. Try to hit each boost pad, especially the one at the end. Line yourself up with the center and jump off the ramp. If you aim correctly, you'll get the third Green Star and many coin rings. At the bottom, Plessie lets you off on the beach near a pipe. The pipe takes you to the flagpole, which has a boost pad nearby.
The stamp is of Plessie.