Water is a very hard medium to move through with speed and power. The higher you get prior to executing the shooting motion, the less resistance encountered by the shooter's shoulder, chest and torso.
Less resistance translates into more velocity being delivered to the ball.
Practice this without the ball. Try to feel the turn of your hips initiating the motion of the shoulder toward the target. The rotation of the vertical axis is maximized by using the abdominals, chest, shoulder and, finally, the wrist to direct the ball in the same motion. Imagine that your upper body is a whip, and the handle is the hips. Again, practice without the ball.