Agree with the above, visit in the late summer, July, August and early Sept are very hot most everywhere in Florida. Nothing wrong with retiring there but many people move here without realizing that winter vacation becomes work when here your round.
I would do as suggested, contact a local realty company and find a rental, maybe 6 months in one location and 6 months in another (if they will do a lease that way). If not then 1 year in Marathon or Big Pine. A realtor can also give you data on property taxes, home insurance rates (make sure you can even get hurricane insurance). Also being there during some of the holidays can be a little aggravating if you need to get somewhere. I have seen traffic backed up for miles over Memorial Day weekend.
Also if he is fishing are there things you want to do? Do some research for yourself too! Its a way of life that many people love and others only last a year or 2 and then leave. We love the keys too but like to keep it as a special place to visit.