The important thing to know is which beach? The closest one to the convention center would probably be Davis Island which is about 4 miles.
If you're looking for a really nice beach. St Pete Beach, Ft. DeSoto Beach & Clearwater Beach are approximately 50 to 60 minutes from the Convention Center.
I did not visit the beach during my visit, sorry
The closest beach is as said before Davis Island roughly 4 miles from the convention center
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