Surprise Shinobi Slippery Splash-n-Spray Sports appears in Senran Kagura: Estival Versus. It's an event created by Haruka as a means of coaxing Sayuri to reveal her true intentions behind the girls participating in the Kagura Millennium Festival.
A ritual combat passed down among shinobi since ancient times. Shinobi compete to see who has the mightiest rage while suspended over a poisonous, watery pit known as a pool. After the battle, the winner is rewarded for her dominance by being presented with the losers' clothes, while the losers themselves stand drenched and fully exposed. This part is known as the "Ritual Slip-Out," and is where the event gets the "Slippery" in its title. ...Or so we've been led to believe. In reality, this was just an event Haruka thought up.