how to do this effect

What does the rest of the frame around the drop look like? Is it filled with a pattern or just one shade of bright or dark tone? If the latter, try changing the compositing mode from the "Modify" tab at the top of the screen. Drop on upper video track, new background below. Highlight upper track of the drop, select compositing mode, examine results. if you're lucky, this may be all you need. BTW, there are great free clips of water elements in FCP, Motion, and Livetype, go browsing what you already have using the inspector tool, it might work for you.