Since access is limited, reservations should be secured well in advance. Boats leave each day at 9 a.m. and 11:45 a.m., with two return options during spring through summer. Come prepared for a full day of beach and hiking with food, water, sunscreen, bug spray, etc. These things are not available on the island.
Cumberland Island is maintained by the National Park Service. Access is limited per day and only available via ferry from St. Marys, Georgia.
If you visit during the summer, I would definitely recommend the 9 a.m. ferry, so you can beat the heat of the day.
We visited in spring because we are well aware of how hot summers are in the South. Spring provides mild temperatures and few thunderstorms, making it an ideal time to visit.