You should know that May is the middle of upwelling season along the Central California coast. Winds push the coastal surface waters south and west, and the deep, cold, nutrient-rich waters replace it, making for low visibility. This doesn't mean every day at every location has bad vis--just that there's a greater chance of it than during winter.
I don't know how far south the upwelling goes, so So Cal may or may not have better vis in general.
Upwelling season also means lots of whales in Monterey Bay, so if you have a chance you may want to do that, too