I told my dentist how much I liked my water flosser, and he immediately said "that's fine but it's not very effective, you still need to use regular floss too".
I use both, and I can say that I definitely get more out from between my teeth with the water flosser. These days string floss is just for when I have something big stuck tightly that I know the water flosser won't be able to shift.
Meanwhile my dentist said I should be using a water flosser instead of regular flossing.