While I was browsing through a few random nail blogs I stumbled upon this technique for creating a marbled nail effect using clingfilm (link to original post containing the technique here!).
I used Models Own Snow White as my base colour and H&M's Blue My Mind - a bright cobalt blue over the top. To get this effect I painted 2 coats of Snow White and let it dry completely before applying one relatively thick layer of Blue My Mind nail by nail. Before this dried, I used small pieces of rolled up clingfilm and gently dabbed some of the blue colour off the surface so the white polish could be seen through.
I really like this effect and although I haven't tried to I'm sure another colour could be layered on top of the blue (I'm thinking pink or yellow). This gives a very different look to water marbling but takes far far less time and with none of the mess or hassle.