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Put "South Beach Miami" in your subject line. There are lots of beaches in the US, and people who know what you want to know may not bother to open your message because it is too vague.
Yes, you can walk the beach at night. Safety is an issue though. Plenty of intoxicated people to be concerned about, just depends on which stretch of the beach.
Technically the City of Miami Beach closes the beaches from midnight to 5am, but unless you're doing something suspicious, they (Patrols) won't bother you....but you do need to be alert in some areas at night...