Make a new layer, and taking a Hard-edged Brush (), paint on splotches where you want water drops to be. It doesn't matter what color.
3. Fill Opacity
From the layers palette, select Blending Options (). Adjust the Fill opacity so it's at 0. The picture should look just like how we started now.
4. Drop Shadow
The rest of this tutorial will entirely be done in the blending mode window, so you might as well keep it open. Go to Drop Shadow, and adjust it like so:
5. Inner Shadow
Go on to inner shadow.
6. Inner Glow
7. Bevel and Emboss
Also, you can select the "New Style" button. And name it Water Drops. Now, all you have to do is select Water Drops from the layer style list, and skip all this hassle next time.