The water jetpack in Super Mario Sunshine is FLUDD, short for Flash Liquidizer Ultra Dousing Device. It is a robotic backpack created by Professor E. Gadd and given to Mario at the beginning of his adventure on Isle Delfino.
FLUDD is more than a jetpack: its water tank lets Mario clean graffiti, defeat certain enemies, activate mechanisms, and reach distant platforms. The default Hover Nozzle briefly keeps Mario airborne, while the Rocket Nozzle launches him vertically and the Turbo Nozzle gives him exceptional speed across land and water.
A common mix-up is calling the whole device the “Water Cannon” or “Squirt.” Squirt is the name of one of FLUDD’s nozzles, not the machine itself. FLUDD is temporarily removed in some secret stages, requiring Mario to rely on his normal jumping and movement skills. The device is central to Super Mario Sunshine’s water-cleaning gameplay and distinguishes it from earlier 3D Mario adventures.