Or, if you're female and need to do this, cross your legs and kiss your companion. Then, it's impossible to see what you're doing, and if you're still under the spray from the showerhead, it'll be indistinguishable from the rest of the water. This may be adaptable for use by males but i think you'd have to do something to make sure you stay aimed at the shower floor, as pissing on her leg may be a dead giveaway if she's at all observant.