Step one:
Open background image. Here we used 400 X 250 pixels at 72dpi.

Step two:
Create a new layer. Press “D” to set default foreground and background colors. Filter > Render > Clouds.

Step three:
Filter > Stylize > Find edges.

Step four:
Now adjust the levels for more definition in the image: Image > Adjust > Levels (Cntl L)

Step five:
To create the drops: Filter > Sketch > Plaster. You will have to experiment to find the best settings for your image size and intensity of the effect, here we used: Balance 40, Smooth 14, Light - Top Left.

Step six:
The result should resemble this.

Step seven:
To define the edges a bit more: Filter > Sharpen > Unsharp Mask: Amount 500, Radius 1.0, Threshold 0.

Step eight:
Select the magic-wand tool, Check the contiguous box on the status bar if using Photoshop 6.0, otherwise it is in the options palette. Set the appropriate tolerance setting, we used 10. Now select the background around the drops.

Step nine:
Hit the del key to erase the unwanted background. Invert the selection to the drops. Select > Inverse or Cntrl + Shift + I. You should now see the original image and the drops. If you wanted the image to appear as if mercury had been spilled on it you can stop right here. For the water - proceed...

Step ten:
To make the drops look more like water change the layer blending mode to soft light. We now have an image that looks like water drops on it. But lets have some fun and give it that lens-type distortion typical to real water drops.

Step eleven:
With the drops still selected, click the background layer on the layers palette to activate the background layer. 12/ Make the distortion:

Step twelve:
Filter > Distort > Spherize. 35% (experiment to find what best works for you.)

Step thirteen:
You now have an image that looks like it was left out in the rain with realistic water drops.