Thanks for the reply Waste, I should have posted my numbers. I've got about CYA 50 so 15ppm is the normal shock. (I should have checked the chart before I posted... ;-) I was really only intending to shock it to the Pool Forum Approved numbers and I mistakenly thought it was 20ppm when I posted.

It's not unusual around here to have a storm fill the pool to the tops of the skimmers and I was thinking I would get in the habit of shocking it then to brighten the tile, since I'm already going to have to add chlorine to the rainwater.

I've re-thunk my position and I think I'll just put the beer on the edge of the pool and swim around and use your suggestion of watered down bleach and elbow grease.