You came to the right place. MJ generally has more informed posters than the reddit that you posted on previously.
Unless you live right along the river, along a creek, or at the beach, you should have no problems with flooding in Jacksonville.
I don't have personal knowledge about Internet Marketing, but I've seen and read about a lot of firms here, so it shouldn't be to hard to find a job. Although this is true of most of non-rural America right now, Jacksonville's ecconomy is booming right now. My wife is an RN and we moved here soon after she graduated. She had no trouble finding a job with good pay at a local hospital.
It cannot be understated how much living in St. Johns county sucks. Want to sit in traffic for over an hour to get downtown? St. Johns is the place for you. The population is growing rapidly yet the road infrastructure connecting St. Johns and Duval is more or less the same as it was 20 years ago when a quarter as many people lived down there. The schools are rated well, but the actual education is mediocre (in my experience). The phenomenon is easily explained when you look at the income demographics for those schools. When the 25th percentile come from a household with +$100k income, it's not hard to get good test scores. Duval has some very good public schools in certain areas, excellent magnet schools, and excellent private schools.
The cost of living is quite low here, especially if you plan on buying a house. Renting can be expensive in parts of the city most apartments here are not ideal for raising a family. Most college educated two income families here have plenty of disposable income after living expenses. While there isn't as much to do here as a larger metro, my wife and I never run out of things to do; in fact, we have trouble finding time do everything we'd like to do.
If you have any questions about specific areas, feel free to ask.
