This is an interesting question. So there is a lot of plastic floating in specific areas of the ocean, due to tidal and meteorological weather patterns. In order to know if there was more water than plastic, we would need to know a) How much water is in the ocean , b) How much plastic is in the ocean and c) plastic > water in the ocean. The problem in doing this equation is that we don’t really know with certainty a) or b). I hope that makes sense to you and shows, how as scientist we break down the big questions into smaller parts, try to answer those and build them back together to give an overall conclusion.