The concrete structure you're looking at is a remnant of a mast house, which would have once supported a large crane. During World War II, this site was part of the Officer's Candidate School for the Merchant Marine. Candidates trained here using and unloading cargo with various cranes and winches. You can still see "cleats" — the metal hooks, like those on the top corners of the concrete block — which were used for securing lines.

Interestingly, this training also involved "barrage balloons." These large, blimp-shaped balloons were deployed over ships to protect them from low-flying enemy aircraft. Their cables could snag planes, or simply force them to fly higher into the range of anti-aircraft gun fire, making them an important defensive measure when ships were most vulnerable during cargo transfer.