Plugging a granite node (P5 and later) into the specularity value socket can also help, especially if you dial down the scale considerably. Not recommended for close-ups though. Also, you can use the same granite node to add a tad of displacement - sometimes helps.

Alas, there is no way (with the current technology) to "simply" add a wet look.
The problem comes from the complexity of what's going on to cause that look (in real life).
Water tends to spread out as a darker layer on horizontal surfaces but gather together in separate dribbles as the surface becomes closer to vertical. The flattish bits are thin and the dribbles are not so thin. All (well, mostly) caused by the water's surface tension. And there's no set of nodes (that I know of) that can calculate all that plus add the required depth and transparency/specularity (or refraction/reflectivity) to all parts of the skin, any part of which may be orientated differently and may have different oilyness...

The only way I've ever come up with anything that looked remotely reasonable in near-closeup was to create an extremely detailed and specific set of bitmaps for specularity, transparancy and bump - and specific to a single character and the pose. The whole thing took me nearly three months to complete (admittedly in sporadic sets of enthusiasm) but finally I just junked the lot after my HD gave up the ghost and I couldn't face the effort of trying to re-create all that I hd done..

Sorry to be the bearer of bad news:-((

If you ever find a way of doing it, be sure to post here - there are several hundred others waiting to hear for you..:-))

Cheers,
Diolma