I've always found it much easier to get into Fort D than the state parks in the Keys. I've also found October easier in Florida because it's still technically hurricane season and can be hot.
If you want to check in on a weekend date, you'll possibly find many waterfront sites already booked by weekending locals. But if you are willing to arrive on a weekday, your chances might improve.
It might be tough to get two waterfront sites side by side, but if you are willing to split up, your chances again obviously improve. There are pull-through sites in the center (not on water, but across from waterfront sites) that seem to stay empty often.
I'd put in for vacation time and just research some other options you can easily book six months out if you can't get exactly what you want.
