I've seen research data that says a water deficit does make apples sweeter. My firsthand experience is rather limited in that I don't have the wet vs dry comparisons like I've had with stone fruit. And maybe I'm wrong but those big beautiful apples in the store just look over watered compared to my best fruit. I can't forget the small Pink Lady I've grown that were so sweet and tart. So much better than the twice as big store bought fruit. Only thing is I can't prove even to myself that the difference was mostly water.