South Beach is on Miami Beach, a separate city across Biscayne Bay from Miami. Every route is a causeway.
From downtown, Brickell or the port: the MacArthur Causeway, about five miles. Twelve to fifteen minutes mid-morning, forty to fifty in the two hours either side of six on a weekday.
From Wynwood, Midtown or the Design District: the Julia Tuttle, about six miles. Fifteen minutes off-peak, thirty-five to forty-five at rush hour.
From MIA: roughly twelve miles and thirty to forty-five minutes depending on the hour. Fort Lauderdale's airport is a genuine alternative for this neighbourhood and is often no slower.
No rail crosses the bay. Metromover is free but covers downtown and Brickell only; Metrorail does not reach the island. The connection is a rideshare, a bus or a drive.
Once you are on the island, the free Miami Beach trolleys run the length of it at roughly fifteen to twenty minute headways, which is the cheapest way to move between the beach, Lincoln Road and the bay side.