I absolutely love this thread!
I recently shot some macro of oil drops floating on water and here is my set up and the shot.
OK, I just set up the oil drop macro thing again just to show you exactly how I shot it. Sorry about the lousy quality of my shot...I took it with my cell phone.
1. So I had a glass dish sitting on a sheet of glass or just a large flat Pyrex dish and figure out how far your subject has to be from the water surface. I did it "dry" by measuring the distance from my 100mm macro lens with some cheap extension tubes 65mm in total for the tubes (no glass inside them). I used 2 rulers: one for focusing on a number on the ruler and another one for my husband to measure the distance from the lens to the number I had focused on. It needed to be about 5 inches away from the water surface.
2. I placed a colorful picture on the floor between the supporting boxes and planted an off camera flash pointing a t the picture. The brighter the image, the better.
3. Now, I positioned the camera on a tripod about 5 inches away from the subject which wasn't even in a dish yet. I got my remote shutter ready, in my case an app on my android called EOS Remote, free from Google Play Store or you could simply use any remote shutter.
4. Have the camera face the dish, carefully pour water into the dish about 5 inches away or whatever your calculations give you and place something on the surface that will be used for focusing. The DOF is super shallow (I shot at f2.8), maybe 2-3mm which is 1/16" maybe. Razor thin. I used a fruit sticker that floated on the surface because any paper placed on water got saturated within seconds and sank to the bottom. You can't focus on that.
5. I had a bottle of oil ready with a long bamboo skewer for picking up some oil and dropping carefully a few tiny drops of oil onto the surface. My remote shutter was ready as was the flash. I kept shooting and live viewing the results.
The oil drops have a mind of their own and floated where I didn't want them to float. I carefully dragged them into place using the same skewer. Some drops merged into a large blob which I didn't want either, so I cut them in half with that skewer and moved one away before they "decided" to merge into one big one again 
I have attached an image that shows you my set up
If you have any questions, please ask.
Forgive the mess; it's my art studio where I paint, teach and shoot 
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