Q:

Why does a street light cast a wavey shimmer across the water (at night)

You will have to zoom in.
Why is the reflection a wavey one? 
Could someone explain how this is occurring, to the layman, (with the style of feynman if possible -heh) :)

A:

If you live by a lake you must have noticed that at windless nights ( and days of course) it becomes a mirror. Street lights are reflected with the dimensions they have.
If/when a breeze starts the surface breaks into waves. The nature of liquids is such that the equations describing the small disturbances of the surface are wave equations. The reflective property of the water is retained and the street light turns into a long wavy line of light, because the water becomes small displaced mirrors, with a direction dependent on the direction of the wave.
On a street when there is  enough water the effect is the same. If it is just wet, the surface irregularities  will generate many small mirrors that will change a spot into different shapes, dependent on the irregularities of the terrain.

