Spoken like a true non-surfer!
Rain - especially early rains after a long dry spell - brings tons of Sh!t into the ocean - literally and in the broader sense. That and the fact that the waves are usually a jumbled mess due to accompanying winds means only the most foolhardy will go surf in SoCal during or immediately after a rain.
Besides. You wouldn't want me to get wet, would you?
EDIT:
Just checked Surfline. About ten people out at Lowers and six out at HB - north side of pier. There are decent-sized waves. Pretty crumbly and lumpy, but rideable. So it's just the septic factor to consider. Tradeoff with the low crowd factor.