See 31.
Oh I’m not arguing about what makes sense on a beach, just commenting on the idea that the virus somehow knows what 6ft is. It doesn’t. It’s just an easy approximation of a distance where respiratory droplets don’t typically travel when someone is coughing, sneezing, or talking. Since respiratory droplets are the primary transmission vector for coronavirus, it makes sense to use the limit of the distance they typically travel as your advice to the community about how far apart you should stay from people not in your household if you’re looking to reduce transmission.
For what it’s worth, I don’t think a beach is nearly as risky to have people as - say - Costco on a typical day. Latest I saw from CDC said indoors is 19 times higher transmission risk than outdoors. Beaches being open shouldn’t make much difference. 2,000 of your closest friends packed into Costco fighting over toilet paper? Yeah, that’s a problem.