Snag a Cannon Box from the start and destroy the statues for some coins, then take the pipe to the next area. The ?-Blocks in the air will expand and make some bridges, allowing you to get on the walls, although nothing of note is on them (except a Super Mushroom in one of the wooden crates). Get in the water and begin moving to the right, avoiding the Madpoles, Bloopers and Rammerheads that you encounter along the way. There’s nothing inside of the clear pipes, but once you reach the end of this area, you will find a large brick wall. Blast it open and grab Green Star #1, then swim up to the Warp Box.
You will have the Checkpoint Flag before you, so touch it and get back in the water. This section will have some Bloopers, as well as spiked blocks. When you get to the point where you have to swim downwards, go upwards to a small area that contains Green Star #2. Swim down the previously mentioned path and blow up the brick wall there to uncover a Green Star Ring. You can actually use the cannonballs you fire to pick up the coins, which makes it much easier to get all eight in time to net yourself Green Star #3.
A little more to the right, dodge past the Rammerheads and blow up the first wall to find a clock behind it. You should see a pipe at the end, but this brings you to the bottom portion of the Goal Pole area, where you can’t get to the top of the flagpole. Instead, there is a brick wall that needs blasted open right next to the pipe, where a clear pipe can be found. This will lead to another area with a pipe, which will bring you to the upper area of the Goal Pole, making it easier to get to the top of it.