The polishes I'm going to use are Ulta3. I've been able to get some Ulta3 polishes to work for a watermarble, but they were brand new. The polishes below are not brand new, they are all at least a year old, some even older.
Ulta3 polishes rarely spread in the water for me, and if I do manage to get them to spread by wobbling the water, then I can't get a nice clean drag through the polish. It always grabs and pulls lumps through or leaves huge gaps. This was the best I was able to get in several trys.
The trick that has worked for me is the same one that I used for the neon polishes. You need to thin them out. For these polishes again I added a full dropper full of thinner. In the pink polish, which was half empty and a bit dried out, I added nearly three droppers full. Again you have to really shake them to make sure the thinner is mixed in really well. You are aiming for a consistency that will drip off the end of the brush quickly and easily.
This is easily the best watermarble I've been able to create with ulta3 polishes. Although the ulta3 polishes still have a tendancy to separate when you drag the needle through., which leaves fine gaps that will show you base colour (white in this instance).