We went last year from the 14th - 18th. Crowds were MUCH lighter than sites like Touring Plans suggested. Considering the southern schools go back in session right at the beginning of your dates...it will cut crowds quite a bit.
Your bigger concern will be handling the heat. I doubt you will see many 90+ degree days in Maine, but if you happen to have a heat wave...you might try using it to help condition yourself for the trip to FL. Beyond that you need to stay hydrated- water with you at all times. A misting fan and or cooling neck band might be helpful. Plan for air conditioned attractions and shows at the hottest times of the day. Consider walking sandals vs sneakers as cool feet can really help keep your whole body cool. Wearing a hat is another big must for us that a lot of people forget or bypass...i.e. a light colored baseball cap will do. Timing it all would also be good. It won't be as hot in the early morning hours and any park with evening EMH will be your friend. Thankfully, we saw a number of nights where MK and Epcot were open very late! Plan downtime at the hottest times...i.e. either a nap in an air conditioned room, a trip to the pool or a trip to one of the waterparks.
Have a great trip! :wave: