use heavy duty PVC pipes as vertical spacers on the inside edge of the boat slip. This keeps the boat from getting under the edge of the dock no matter what the water level. Drive the pieces down into the mud about 18 inches apart, and bolt them to the top edge.
I posted some pictures on a different thread a while back; I did a search and couldn't find it, and I don't have the pics anymore. Adam, I think it was on a thread when you were building your dock.