Water Drops:

1. Pattern
Open up the file that you want to add water drops to.

2. Splotch

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

Inner glow...

7. Bevel and Emboss


Final Thoughts:
Be sure to use a hard edges brush, or else it won't work. The key factor here is the Fill Opacity. The Fill option makes the real paint be transparent, while layer effects still show. A round of applause for Fill...

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.