2 questions

Specular maps!

Seriously what kind of vague question is that? It really depends on the engine / tech you're using. Wet surfaces are achieved via shaders with a mixture of spec maps, cubemaps for reflections and alot of shader tweaking.

I have a small tip: When it's wet surfaces tend to be darker then when they are dry. So take that into account when doing/tweaking your diffuse maps/materials. Also reflectivity and gloss is often higher on a wet surface. But don't go overboard with it and consider the surface type of your material. On some stuff it just looks like a layer of slime. Study some real world references.