Shadow of a ripple?

You can actually think of the ripple as a travelling lens.

If you take a radial cross section through the ripple, it'll have a curved profile. Now just like a magnifying glass causes a bright spot in the middle of where you focus incoming light, it also causes a darker region around it.

This is what you're seeing on the bottom of the pool: The band of focussed (or unfocussed, depending on depth) light caused by the lens-like nature of the ripple wave.