I you are looking for a nice beach like you would find on the coast line farther north, you are going to be disappointed.
With that said, my wife and I love John Pennekamp and have two two week stays their this winter.
We stay there several times every winter.
There are two small beaches both with hard packed ground coral type sand.
"Near beach" is about a quarter mile walk from the camp ground, and "Far beach" is a few hundred yards farther.
Neither one of them is worth writing home about.
On the other hand if you are a canoe or kayak paddler the place is fantastic.
You can rent canoes and kayaks at the boat basin, and there are canoe trails through the mangroves.
If you have your own kayaks there are enough good paddling places to explore for as long as you are there.
If you do happen to be a paddler, let me know, and I'll send you as many trips as you want.
You can take dive boats or the glass bottom boat from Pennekamp to the off shore reefs but only expect to see small fish at the beaches at Pennekamp.
Hope this helps a bit.
If you need any more info, e-mail me. We have been going there for years
Jack L