Symbolism, Imagery, Allegory
The kids have to learn how to scumble—a.k.a. control—their savvies in order to be out and about in society. Since this takes some time and practice, scumbling is a symbol for the self-control people develop as they grow up that enables them to best use whatever powers they possess (whether magical or not) and be happy and healthy adults.
Fishin' and Hopin' and Prayin' and Dreamin'
Fish offers to clearest example of the importance of mastering scumbling and the benefits that can follow. At the beginning of the book Fish is not in control of himself. The whole family had to pick up and move to smack-dab in the middle of the country just so Fish wouldn't accidently start any impromptu hurricanes. Yikes.
And the thing about Fish's savvy is that it comes bursting out when he gets emotional. So any time someone angers him or threatens a member of his family, out come the water works, so to speak. Needless to say, having a savvy is a major adjustment in his life:
After turning thirteen, Fish had never stopped needing to work extra hard to let his own particular color shine through all his dark storm clouds. Having a really powerful savvy like his was similar to waking up with a savage temper: It required a lot of extra effort and patience to control. (21.7)
Scumbling isn't just a matter of convenience for Fish—it's pretty much a matter of life or death. After he learns to scumble though, Fish basically has a new lease on life. He no longer allows himself to be controlled by his savvy, nor does he cause damage when he uses it. He even finally comes to a place where he feels like he can be around water again:
Fish, taking a deep, deep breath like he was setting aside an entire year's worth of dread, jumped into the water with a big, splashy cannonball and the boys launched into a friendly yet frighteningly powerful water fight […]. (25.18)
By learning to scumble, Fish has finally gone from being like a fish out of water when it comes to his savvy to being, well, a Fish in water. Literally. (Sorry—that was a horrible joke. We know.) And since savvies symbolize growing-up in this book, scumbling also represents characters coming into their own—so when Fish leaps into the pool, we know he's going to be okay with himself from now on.