The act of running and jumping in a flexed position into a swimming pool to create a large splash, mimicking the flight and shape of a cannonball.
The New Yorker, 30 August 2004, p.40
To jump/dive into water doing a cannonball landing.
He cannonballed into the pool, drenching us all.
To careen; to move rapidly
Yelled when jumping/diving into the water, doing a cannonball landing.