If your going to stay inside them US, South Padre Island, Tx. and Islamorada, Florida Keys are the only 2 options. Some one else will tell you about South Padre.
Islamorada, Florida Keys - Winter temps average 70 - 85 degrees. Water temps run from mid-60s - upper 70s. Cold front are common from November - early March and temps can fall into the mid 50s but not common and only lasts a day or two. February - May gets a lot of easterly winds which are warm and steady. There are hundreds of miles of shallow (waist deep) water to learn and ride. Lots of great riders and great people. In case of no wind? This is the Keys! Diving, snorkeling, fishing, sup, sailing, nature, drinking, I could go on. Key West is 1 1/2 hrs. to the south and Miami is an hour North.
If this sounds like what your looking for, call Otherside Boardsports. All instructors have a minimum of 5 years teaching experience and come with top credentials. Some have been riding for as much as 14 years. A few pros also work there. Otherside is the only school and shop with it's own cable park (Keys Cable) to learn how to ride the board before your kite lesson which will greatly speed up your learning curve. It is what is called a 2 point system which is catered specifically to the rider.
Lots of great places to stay depending on your budget. From $50.00 - $600.00 a night.