Water Ball

Water Balls are objects in the New Super Mario Bros. sub-series. They are large masses of water that float (and sometimes move) in the air, and are shaped either as circles or ellipses (horizontal and vertical) and look similar to bubbles.

New Super Mario Bros. Wii
Water Balls first appear in New Super Mario Bros. Wii, where they are found only in World 7-2. If the player jumps into one, they will start behaving as if they were underwater. This allows the player to swim inside the Water Ball until exiting it, at which point the player will fall 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 also travel slower when they go through a Water Ball.

New Super Mario Bros. U / New Super Mario Bros. U Deluxe
Water Balls return in New Super Mario Bros. U and its port, where they are found only in A Quick Dip in the Sky. They behave the same as in New Super Mario Bros. Wii. Koopa Troopa shells are also shown to travel slower while moving through a Water Ball.