crank up contrast(maybe brightness too) a lot.

apply transparency map and fill so it's transparent: Airbrush varyingly opaque bits vaguely over the highlights, but liberally sprinkle all over the place.

Add noise filter to this. Merge down. Add a couple of bigger 'drop' style highlight.

Voila. Wet skin.

See the Electra pic on Imaginos's site for an example, or see attached.

Hope this helps.

Mini Tutorial by Bigboote, Picture By Riv.