Before WWII, the Panhandle and Northern Florida had your typical Southern culture (even plantations and slavery, antebellum). Everything south of St. Augustine was rather sparsely inhabited and not a very healthy climate. Electrification and air conditioning made it more liveable, and Central and Southern Florida were massively swamped with Yankees – not just retirees, but families seeking economic opportunities and better quality-of-life. There are still plenty of Cracker families in Central and Southern Florida – my late mother’s, for instance – but the Southern drawl sounds just a bit exotic here by now. Come to think of it, it even sounds rather exotic in Atlanta. By my grandparents’ standards, Bill Clinton and Al Gore sound like Yankees.
Before someone gets upset about BG’s use of the term “Cracker”, let me 'splain that in Florida (at least when I was growing up there) the term is not necessarily pejorative. I remember in elementary school when they taught us the “I’m a Florida Cracker” song.
Florida Land Boom of the 1920s. Even then, Miami was a draw for New Yorkers.
As I have said several times on the SDMB, this is the best study of the cultural and economic regions of North America:
Northern Florida is part of Dixie, while southern Florida is part of The Islands.
Basically the “true South” stops about 10 miles north of Gainesville, and picks up again just south of Kissimmee, all the way down to the south side of Lake Okeechobee. There are a few Southern pockets here and there in the “I-4 corridor”, but anything within a throw of I-4 is generally “Northern”, at least in my experience (lived here 30 years, been all over north/south-central Florida).
Gainesville isn’t Southern? That would come as a surprise to Gainesville native Tom Petty, I think. According to Tom:
There’s a southern accent, where I come from
The young’uns call it country
The yankees call it dumb
I got my own way of talkin’
But everything is done, with a southern accent
Where I come from
If you stay out of University of Florida and the student ghetto surrounding it, Gainesville is southern.
Let’s talk about what the word “most” means. 
IME, everything west of I-75 in Gainesville is southern - everything east of 75 is UF.
The panhandle is known as the “Redneck Riviera” for good reason. Jacksonville/northeast Florida is really just south Georgia.
Depends… I worked and did errands in parts of Gainesville east of I-75 that were definitely southern.
Look! I made a map for another thread like this one. I’m open to contributions, but I’ve been all over this (damned?) state, and that’s what I’ve experienced.
An excellent book on this subject is Frog Smith’s A Cracker History of Florida .
Outside of Florida this may be a difficult book to find
just go into a ihop/dennys/other, if the waitress calls you darlin’ your in the south… 
That is impressive, Bambi!
Every time I’m in Miami or any of the surrounding old-people colonies, I feel like I’m in Noow Yawwk or Noow Joysey. A bunch of pushy old bastards driving gigantic land-yacht Lincolns and Cadillacs everywhere, wearing white golfing pants and huge eyeglasses. They drive horribly, and shout at each other all the time. I guess at night in Miami there’s sort of a night life, but it’s not the kind that I really go in for - there’s something very tawdry about it, like it seems somehow artificial and insincere. I definitely wouldn’t describe the atmosphere as Southern.
Yes, but her culture shock may be from the diference between country or small-town living and city living. I experienced “culture shock” when I moved to Atlanta, but it wan’t because the city isn’t Southern. It’s just because it’s a city.