Sounds like nonsense to me. I am not aware of any beach where people are not allowed in the water due to COVID. Nor does a search of news stories reveal any. Can you point to an example? What beach is this tweet from? Of course the tweet is from a Twitter account that simply spreads conspiracy theories about COVID.
There are numerous beaches for water entry closed due to pollution/waste plant spills, aquatic issues (plant/animal threats e.g. jellyfish), weather/surf conditions, or not having lifeguards staffed past Labor Day. Not a single one closed for water entry due to COVID.