The Spanish word "aguacero" means "downpour." If you break it up, it seems to literally be "water zero." Is there anything to this? Is the etymology here implying that so much water fell to earth that there is zero left in the sky?
Short answer: It does not mean water zero.
As guifa commented, it comes from agua and the suffix -ero, connected by the infix -c-.
Este vocabulario en su etimología se compone del sustantivo “agua” agregando la letra “c” como enlace y de la morfema flexivo “ero”.