You have a choice of two docks when you arrive at Cumberland Island.
The first dock is near the Dungeness Ruins. But that self guided tour
includes three legs: first a half mile walk through lose pack dunes,
then a mile walk on the beach and finally a half mile trek back to the
second dock.

The second dock has a half mile access to reach the beach. Kim and I took
the first dock, the rest of the group stayed on the boat and headed for the beach.