Beach scene location and observations.
Note the amazing beach scene from 26:41==>31:05. As stated in another post, I don't believe you could get a crowd like that together today, even as screened extras. They're also wearing their own beach attire from that era. Although many of them are screened and rehearsed extras, it's a pseudo-snapshot of the American people before the fall.
This scene was shot on St. Petersburg Beach, St. Petersburg, FL, just a little south of the Don Cesar Hotel. https://www.loewshotels.com/Don-CeSar-Hotel
I'm guessing the cast and crew stayed here while in St. Pete.
Here's an article about the making of the movie in St. Pete:
http://news.google.com/newspapers?nid=950&dat=19850808&id=qcta AAAAIBAJ&sjid=b1kDAAAAIBAJ&pg=5814,1843429
According to the article the beach looked unusually crowded. Maybe ads were placed and posted asking people to show up certain days with their families.
For contrast here are some views today:
http://beachcrawl.blogspot.com
Note how stocky the people are, even though not obese.
http://stpetersburgfldailyphoto.blogspot.com/2012/05/st-pete-beach.htm l
http://www.theroaddogblog.com/2012/03/saint-pete-beach-florida-photos. html
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The story is king.