I did what you are planning to do, a couple of years ago and was basically told to go to the Keys for my diving.
As one dive store operator put it:
"You have to go about 30 miles out into the gulf to get anything interesting to dive on with decent viz. This means your trip will take about two hours out and two hours back with the average dive boat traveling at 15 miles an hour, and if conditions change on the way out, you will be doing a 2 hour boat trip for nothing. It is much easier to just drive for an hour and a half over to the keys where you are almost virtually guaranteed good diving every day."
As I was in disbelief about his reply, I further inquired as to where they train their new student divers. His answer was, he makes all his students drive to the Keys for their lessons.
Naples and Marco Island are really beautiful places to vacation. I was glad to have had the opportunity to vacation there, and was only disappointed by the lack of good diving.
Not sure what the diving is like north of Naples, but would be really pleased if there were some great sites to dive, as most of the family prefers to go to the gulf side of Florida around Naples, Clearwater, and\or Fort Meyers area for the winter.