Nobody really knows that answer. William Morgan invented the game of volleyball in the 1890s and it is known that beach volleyball started in California in the 1920s. But who played the first game or created it, history has not recorded.
The first beach volleyball was played in Santa Monica, Californa and the rules were made by people pretty much copying indoor volleyball.
Whoever did was amazing and a hero to man kind. We will thank him forever!
Let's just put it this way, we were playing team beach volleyball on the beaches of Perth, Australia back in the 1960's, and Australia is not really a 'volleyball' country, though it certainly get into its beaches!