Since you're from Boston, I assume that you want this catamaran to be your winter home down South. In that case, anything further north than Stuart, Florida is going to be too cold in January and February for you to enjoy it very much. In Georgia you will get frost, (rarely) snow, and generally a bit too cool during the winter months for swimming.
I wish I knew what you've been told about what is cheaper about keeping a boat outside of Florida. Florida is a bit aggressive about collecting taxes for people who stay there on their boats, but sales taxes paid outside of Florida count against any sales tax that might be due to the state. Registering the boat in Florida every year isn't unreasonable - $75 a year? Compare that to what you've just paid for a 40 - 45' boat?
There's a lot of year 'round boating in Florida, and with 1,197 statute miles of coastline, you've got a lot of exploring to do without getting bored. Plus rivers, bays, and islands on both sides of the state, all told about 11,000 miles of rivers and waterways. Too many choices to cover in this answer.
Florida has so many marinas at prices from ridiculously expensive to sometimes quite cheap. A catamaran might be very expensive to keep in a marina because of its width, though when just on a face dock, it usually costs the same as a monohull. Some places that are reasonable are in Stuart. My experience is with Sunset Bay Marina in Stuart, which offers marina berthing and moorings. The mooring is quite reasonable, under $400 per month, and you have full access to all marina facilities including the lounge and laundry. There are others in Stuart, and just about everywhere up and down both coasts. Just about every marina in Florida can be found quickly with an Internet search - just pick a town you might want to visit.
Insurance costs shouldn't be a problem if you don't keep the boat in the water in Florida for hurricane season. Either have it hauled while you're not there, and insurance won't be such a problem. Or bring it north to the Carolinas or even Georgia to haul it or leave it in the water in a marina, such as Brunswick Landing Marina in Brunswick, GA.
I hope this helps. Any more questions, fire away.