I actually don't think it's totally fair to blame the people on the beach. There's a lot of just general distrust in government anyway and then to hear mixed messages over what the procedures are (and it's been a few weeks of this) can be confusing to people to the point where they stop listening.
I think leaving the beaches open only further hurts because with very little else to do in Florida right now, school out and both the CDC and the Federal government recomending against gathering of anymore than 50 people (or 10 in Trump's recommendation), it's more mixed messaging on why the beaches are left open when everything else is closed. People may think they're safe because of that, which is the danger of leaving them open.