From the Super Mario Wiki
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Water Balls are large masses of water that float (and sometimes move) in the air. They are shaped as circles or ovals (horizontal and vertical) and look similar to bubbles. They appear in New Super Mario Bros. Wii and New Super Mario Bros. U. In New Super Mario Bros. Wii, they are only found in World 7-2, and in New Super Mario Bros. U, they are only found in Meringue Clouds in the level A Quick Dip in the Sky. If the player jumps into one, they will start behaving as if they were underwater. This means that the player can swim inside the bubble until exiting it, at which point the player will jump out. Water Balls are usually used to access high areas or to cross large gaps, but they can also be used to avoid enemies down below, thus providing a safer route for the player to travel through. Bullet Bills will travel slower when they go through a Water Ball.