Attention! I'm releaving my biggest commercial secret now! Merge black waterdrops with white background and apply Blur to it. Radius is important, feel free to experiment. I chose 7 px.
And now, hit Ctrl-L to open Levels dialog, and move black and white arrow pointers closer to the middle. You'll see how our blurred waterdrop become sharp and smooth.
Remove white from this layer. You can use the following shortcuts for that:
(this was another professional secret! :-))
Now it is time for nice layer effects. Set layer blend mode to Screen, and use Shadow, Bevel and Inner Glow to make waterdrops look better.
Some more lights and reflections would make our artwork much nicer. Create new layer (with blend more = overlay and opacity = around 50%), select brush tool and white color, and spend another hour putting white dots where light is reflecting in the top of waterdrops (in my case, light is falling from top).