I'm not sure the specific details of ownership, but the gist is that in California, everyone has a right to access the beach. Private land owners aren't entitled to blocking access to the beaches, even across their property.

This court case has been going on for years over this (1) and the only reason it hasn't been tossed out is the billionaire who owns the land keeps paying lawyers to appeal.

(1) https://www.sfchronicle.com/bayarea/article/martins-beach-la...