I know this is Georgia but its pretty close to Florida. ;)
I'm considering a hike in there and was curious if anyone has been there before for an overnight stay, what I am wondering is just how populated it is with hikers. Since you ferry to it with a 7 day max stay I get the impression it could be crowded.
Some friends and I kayaked from Crooked River State Park to Cumberland Island last year. We ran across to the beach and back to make the tides on the way back. Passing the primative campsite we saw not one other person. It was in December (2007). It is an absolutely beautiful place and I highly recommend it.
We stayed overnight in St Mary's last week on the way home from Charleston. It's where the ferry departs - and it was busy! Most of the people were headed to the campground, by the burdens they carried. We stayed in the Riverside Inn because I wanted to, but it wasn't the same as it was on my last visit. It's gotten pretty rough around the edges, kinda like a hiker hostel. But then, everyone staying there pretty much was a hiker, and the prices were good, so ... can't complain! Most importantly, it was clean, it's historic, and still in the same family. But it could use a few million in renovations, not something the average person can take on.
Sandy's right about the ferry and the crowd at Sea Camp, the campground with amenities close to the docks. To avoid the crowds, plan to backpack to Yankee Paradise, Hickory Hill, or Stafford Beach primitive campsites. http://mappery.com/maps/Cumberland-Island-National-Seashore-Map has a good map.