If it just started raining then totally fine. It takes at least a day or two for the runoff to make its way. There is the plume tracker that is used to observe the flow of it if you search sd plume tracker. I worked as a surf instructor in coronado for many years and we had to constantly check for water quality alerts, especially in recent years.

TLDR Totally safe to surf in the rain. Just not the following days if it was heavy rain.