I'm pretty certain there was an article on the subject in woodturning magazine a few months back. I have a feeling though that Terry has it right- it's perhaps marbling inks you need to be investigating rather than paints. I may be wrong but an oil based paint will sink, a water based paint will dissipate, so you need to be looking at spirit based products which are generally stains and inks.
The only paints that float that I've ever come across are attached to the side of a ship