The water here is purple, so don't fall in. There are Piranha Creepers here, Piranhas on vines that will move around if they're not asleep. Stomp on them or use fireballs to make them retreat. If they retreat all the way to the hole they're hiding in, they'll be defeated. You'll need to do this to access the pipe ahead.
The pipe takes you inside a cave. There's a Mystery Box in the back wall. It leads to a room with four sleeping Piranha Creepers and the first Green Star. Bounce on each Piranha Creeper in quick succession to reach the Green Star. Keep going and you'll reach the midpoint. There's some regular water in the cave. Drop down where there are two Piranha Creepers moving over the same platform. At the bottom is the second Green Star. Up ahead is a Piranha Creeper on the wall. You can either ignore it or jump on it and run behind the wall for many coins and the stamp. You can also go underwater and access the stamp from the other side. Go through the pipe.
The pipe takes you back outside, where there is a random item block in front of a purple waterfall. Circular platforms fall from the waterfall and float along to the edge, where they fall off. Cross them to reach two wooden platforms with sleeping Piranha Creepers. There's a Green Coin Ring here. Collect all 8 Green Coins for the third Green Star. The pipe at the end takes you to the flagpole, where there's a sleeping Piranha Creeper in front of the flagpole.
The stamp is of a pipe.