straight styrafoam will NOT hold any water nutrients etc so as such will be a bad idea except for maybe rooting mixed with peatmoss may bell work but need to check if any chemicals are leached that could harm the plant...

The foam floats because it can't absorb any water. If its that nice clean look you want try pure perlite for a rooting medium. It is great. The styrafoam might provide good soil airation when mixed with peat or soil, I'm not sure. I only use it to fill the bottoms of large pots so as to not need as much soil.