Never heard of Marshall(s) Beach either. So far about all I can find about it is:
1. A popular nude beach (apparently with the gay community).
2. One of the greatest spots to view/photo the Golden Gate Bridge, and
3. Also nicknamed "Bad Boy Beach"
One description that appears on the google search page says, "Marshall Beach, San Francisco. A beach in SF named after my family ; ): | See more about Marshalls and Beaches.". However, the link opens a relatively empty/unrelated webpage. But here is the link just in case...
Honestly John M, if YOU don't know the answer to a GGNRA location, it would/may be difficult to find someone else who would, at least in my contacts with family and friends. But I will do more searching as sometimes I get lucky.