No, they did not. Einstein On The Beach is an opera written by minimalist composer Phillip Glass and directed for the theatre by Robert Wilson (not a Beach Boy).

The opera inspired a song by Counting Crows called "Einstein On The Beach (For An Eggman)" which was not written by any of the Beach Boys.

Finally, because Albert Einstein died in 1955, six years before the formation of the group, it is not possible that The Beach Boys wrote Einstein while on the beach -- unless they were very young kids sitting in the sand doing a civics or science project.