Board Bowser's Sub
From the Super Mario Wiki
The Power Star for this mission is located on the deck of Bowser's Sub. To get to it, the player must first swim to the second area of the course through the tunnel, then climb onto the boardwalk along the wall. Pressing the Purple Switch will cause some Blocks to appear, which the player can use as platforms to reach the top of the submarine. The Power Star is on the opposite side. In Super Mario 64 DS, a Bill Blaster is added to the sub.
Collecting this Power Star will open up the way to Bowser in the Fire Sea. Once Bowser in the Fire Sea is completed, the submarine will move out of Dire, Dire Docks through the newly opened hole. However, the Power Star will remain and can still be collected with a well-executed jump from the poles. In the DS version, collecting this star after the submarine leaves is easier by either using Yoshi's flutter kick, Luigi's scuttle jump, or Mario's Balloon ability.
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