Shocking Arrow Lifts!
From the Super Mario Wiki
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To reach the Power Star, the player has to raise the water level to about halfway using the Crystal Tap on the large, stone structure; accessible by using the Purple Switch on the bridge near the start of the level. Once the water level is raised, the player needs to swim north to the wooden floating platforms and use them to jump up onto the smaller platform above with the nearby arrow lifts. When the player stands on one of the arrow lifts, it will begin moving in the direction signified on the lift, then return to its original position. The player needs to use the moving arrow lifts to get to the platform on the other side while avoiding the Amp circling the lifts. Once on the other platform, jumping at the ! Block will produce the Power Star.
It is also possible to complete this mission two other ways. With the water below the platforms, the player can Long Jump to the platform with the Power Star, skipping the arrow lifts entirely. With the water at its maximum height by hitting the Crystal Tap at the opposite end of the course, the player can jump out of the water below the ! Block to destroy it, though this is not possible in the DS version.
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