Aguamenti - The Water Charm
Aguamenti is the charm used to put out fires/ create water to drink. It is first named in Half-Blood Prince, when Harry is being taught how to perform this specific charm in Professor Flitwick’s class. Later Harry casts this spell in an attempt to create water for Dumbledore to drink after taking Voldemort’s potion and then to douse Hagrid’s hut after it is set on fire later.
So your task is to either
a) create something water related, be it colours, waves etc.
b) create something a little more literal like a goblet, something that imitates the jets of water or a fire hose (firemen optional but welcome and yes the professor knows where some minds will take this one and that is also fine :D)
Hufflepuff Water Bottle Sling!