The Deep Shell Well
Fall down the slide at the beginning and grab the green shell. Collect the air bubbles with him, because now you need a long breath. Just follow the path past the monsters and try not to take detours. Pretty much at the beginning of the underwater section you will find a small cave on the left, which keeps a Comet Medal hidden. Grab it quickly and keep swimming. At the end, destroy one of the brown blocks by spin to swim through. Then quickly take the air bubble in the middle, because your breath indicator should already be colored red. In the end, you will find a Launch Star.
Jump down the gorge, grab another green shell and cross the rings. Also, collect a coin for your air display every now and then. Dodge the spiky balls at the end and take the green shell on the far left in the meadow if you don't have any more. Destroy the treasure chest where the Power Star is hidden.
The Chimp's Coin Challenge
After receiving the invitation of the Chimp, you must once again break a record from him in this galaxy. Collect a total of 10,000 points within three minutes. To do this, beam the enemies with your shell so that you get 100 points for each. There are also 100 points for collecting coins. If you’ve already scored more than half of the points before the first launch Star, you’re doing pretty well.
Comet Medal
In the first underwater tunnel, take a Green Shell and search the left wall. Enter the side passage near the Snoodles to collect the medal, then return to the main tunnel.
Green Stars — unlock requirement
Collect all 120 regular stars and defeat Bowser again to unlock the Green Star hunt. Choose the corresponding Green Star mission from this galaxy’s mission screen. Follow the detours below rather than collecting the normal mission’s ending star.
Green Star 1
Swim through The Deep Shell Well to the column-lined area. Near its far end, search behind the columns for the star and listen for its chime. Keep enough air while checking the backs of the pillars.
Green Star 2
Reach the Launch Star and let its sequence drop Mario onto the small ledge. The 1-Up Mushroom is on the left; move to the right side instead to collect the green star.
Check each numbered Green Star on the mission screen after collecting it. The routes finish immediately when the star is touched, including stars suspended over a gap.