This concrete structure was once the base of a large crane. During WWII, as part of the Marine Officer's Training Academy, it was used to practice loading/unloading cargo. Barrage balloons, like the blimp-shaped one shown in the photo, were also used here. These tethered balloons deterred enemy aircraft attacks on ships, forcing them to fly higher into anti-aircraft gun range. Look for the remaining cleats on the concrete block. They were used for tying down balloons.