Well my first thought was “isn’t that an oxymoron?” but it really does describe it. Basically set up your water marbling as per normal but don’t put your finger in! Carefully lay a plastic sandwich bag over the marble and remove.
Leave the bag to one side until it is dry then trim to the approximate nail size (I found leaving a bit spare helped with application).
Apply a thin base coat of polish as a glue and when still slightly tacky put the dry water marble on as you would a nail foil. You should be able to remove the plastic bag and excess dry water marble leaving the nail covered in the polish. Carefully tease away the excess.
Finish up with a shiny topcoat.
Verdict: This is so much less mess than regular water marbling and you can do all the messy stuff in advance making up many bags in one sitting. If you’ve been put off by regular water marbling do give this technique a try. Clean up on the nail is negligible too. This is one I’ll do again.