Using a Waterpik to clean your teeth will usually be good enough to get your teeth clean and you usually do not need to string floss after using the Waterpik.
However I would recommend using a string flossing every once in awhile between using the Waterpik as although the Waterpik should get most if not all the food debris out of your teeth and clean your teeth good enough sometimes you might have a few bits of food stuck in between your teeth that the Waterpik might miss.
When I use my Waterpik I do use the string floss every few days just to keep my teeth clean and I do sometimes find a bit of food stuck in between my teeth that the Waterpik didn't remove even though it's pretty great at removing most of the food debris.