Here is what the Mayo clinic says
Dental floss vs. water pick: Which is better? - MayoClinic.com
"Standard dental floss is the most effective tool for cleaning the tight spaces between your teeth. You can also use dental floss to scrape up and down the sides of each tooth. If plain dental floss gets stuck in your teeth, use the waxed variety. If you have a hard time handling dental floss, try a floss holder. Other options might include special brushes, picks or sticks designed to clean between the teeth. A water pick, also known as an oral irrigator, is a device that aims a stream of water at your teeth. A water pick can help remove food particles from your teeth, but it's not a substitute for brushing and flossing."
