This a topic I have thought much about over the years!
Being Southern is not only geographic, but also hereditary and cultural. You may have been born, raised, and have lived your whole life in the South, but if your parents are both Yankee carpet baggers, you likely aren't a Southerner. However, it is possible or probable that your children will be Southern.
As for South Florida as a whole, i think the cultural factor makes it largely not a Southern place. But there are still native born South Floridians that I would consider Southern. But not as many as you will find in other places in the Southeast.