You may also want to discuss alternatives to water. (Some venues simply won't allow water no-way-no-sir, and they have extremely good reasons for doing so--but when your director insists on the effect...)
Stay away from glitter for what I hope are obvious reasons (you'll never be able to use that costume again, the actor will need to shower to get it out of their hair, it very easily gets into people's eyes [where it stings like the dickens], that spot on the stage is going to glow for the next several years, etc.), but if you're doing slapstick, don't underestimate the comedy potential of a bucket full of confetti.
Confetti is readily available, cheap, requires relatively little clean-up (compared to water, but especially compared to glitter) and is unlikely to cause injuries to your actors. If you cut it chunky (like, half-inch circles) it has the added bonus of being highly visible from the audience.