No trick to get it to float. I have noticed that the thinner the paint is, that it tends to spread out. No biggie, you can still get the same effect.
When the paint is dry, there is no noticeable texture to the marble dip. That being said, if you have any "Globs" of paint on the shaft, it will not dry smooth and flat with the shaft. I had that problem happen at first, so I practiced a couple more times and no texture now. You just need to make sure you don't have any large one spot concentration of paint floating on top. Make sure you have it all swirled together really good. If you continue to have that texrure problem, your using to much paint, or not swirling it on top of the water enough.
I have learned the darker the color, I use only 1 drop. The ligher the color, I use 2 drops.
Take for instance the picture at the top of this page. The pink set was done with 1 drop of black paint. The multi colored set was done with 1 drop of black, 2 drops of olive drab and 2 drops of cream. Make sure they are swirled together nicely and you will be extreamly pleased with the results.
End result...they are smooth enough that you cannot feel a thing when you close your eyes and run your fingers over them. I learned that trick from a Master Carpenter I worked alongside for 13 years (I was extreamly fortunate that he liked me and took me under his wing). He used to ALWAYS come and run his hands over my finished product and tell me it's not sanded enough, you will see the joint. He finally taught me, after having to correct me to many times, "If you close your eyes and run your hand over it, you will feel your flaws". From that day forward he never had to correct me again. No amount of paint or clear coat can cure imperfect work.