Water spotting is a sister-technique to water marbling. You drop polish onto water (as you would with water marbling), but instead of adding another polish and swirling it around, you'd spray some type of alcohol-containing spray onto the polish (ex. body spray, hand sanitizer). This makes the polish break into that spotty texture. It's a cool technique and not hard to do although it may take a minute to figure out the polish/spray ratio you're wanting.
Dark purple (almost black) over red: