From the Super Mario Wiki
The Water Bomb is the Offensive Power Shot used by Koopa Troopa in the game Mario Power Tennis. Much like the Boo Blast, the Water Bomb involves Koopa filling his racket with water and then hitting the ball with intense power. Players who hit the ball after this move is used will slow down temporarily.
This shot is similar to a 0° drop-shot, making it difficult to return. Of course, it is best used when near the net.