Arkiegirl, everyone has given you great advice. Most with prime locations and established rentals will not rent mid week to mid week.
However, as stated, if you can wait till closer to time or look for new listings, or go thru the listings and look for those that have a daily rate listed for summer, you can find some. They are out there. Just have to look hard. Many times though in order to get the weekly rental discount, you will have to go Sat. to Sat. But if you are willing to pay the daily rate, you might get lucky.
Check Beach Rentals of South Walton I know they have some daily rentals during the summer, with a 3 night minimum.
Good luck.