Michael's Genuine and Azul are on the mainland.
I believe Nemo is still closed for renovations.
My rec would be sitting at the bar at Prime 112, but basically anywhere with a bar would be fine for single dining.
Also an outdoor seat on any Lincoln Rd restaurant would be fine for going solo. Some of the best people watching anywhere.
Strangely, there are few restaurants in South Beach with a view, Smith and Wollensky is about the only one.