When asked to photograph products where a water droplet effect is required, we always reach for the glycerin. Rather than use pure water, which experience shows will bead off before you get a chance to depress the shutter, we recommend using a glycerin solution. We always take some time before a photo shoot to get the mixture right: not enough water, and the solution squits out in gloopy globs, too much water and the solution rolls off the surface to quickly. Apply using a fine pump spray about 10-15cm away from the surface, and use wide steady sweeps past the bottle a little at a time. Both the pump bottle and the glycerin are available from good chemists.