Dive is only used to enter the ruins a little East of Undella Town, I believe. The feature is brought back to life, albeit in a relatively bland way.
As for Waterfall, there are a few of them located all throughout the post-Elite Four areas (Eastern Unova). I don't remember the exact locations, but it's a linear path anyway, and are easy to spot. The best you get form the Waterfalls are trainers and a couple items, including a rare candy.