I have no information about the Florida area, but it seems to me that the cart is pretty far in front of the horse dependent on your parents boating experience. You did not say if they are boaters or want to be boaters. This post is only relevant if they have limited experience.
In the Tampa/St.Pete area I believe that you will find quite a few sailing schools and charter companies. Sail and Power. If they want to try the life in a marina for a few days or weeks chartering would seem to be your best bet. Much better to spend a few thousand dollars and sail and sit at a charter companies dock then attempt a private party rental.Liability laws make private party questionable. If they are not experienced and want to learn a liveaboard class is another way, but they will probably be sharing the boat with someone else for that unless they book a private lesson.