Water Bullet was an attack performed by Super Sailor Mercury in Sailor Moon SuperS: Zenin Sanka!! Shuyaku Soudatsusen.
This was a desperation move by Super Sailor Mercury, she executed the move by doing a handstand and propelling herself to her opponent.
The attack was also performed in the video game, Sailor Moon S: Jougai Rantou!? Shuyaku Soudatsusen.