From the Super Mario Wiki
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Leaf Rafts are floating rafts made of giant leaves exclusively appearing in the Starshine Beach Galaxy in Super Mario Galaxy 2. To use one, the player must jump on one, and then stand on the edge of the Leaf Raft facing the direction they wish to go. The Leaf Raft will then begin to move forward, and will continue to do so until the player walks back to the center of the Leaf Raft, at which point it will gradually slow to a stop.
A Pianta in this galaxy opens his "Leaf Raft Shop" for business in the mission Climbing the Cloudy Tower, which is the only time they can be used. The player can use them to cross the large body of water in this galaxy without actually touching the water. This is essential in the aforementioned mission, where the player as Cloud Mario or Cloud Luigi must ride across the water using Leaf Rafts to successfully reach the large tower where the Power Star is located. They are similar to the leaf boats from Super Mario Sunshine.