What is Ship sweat?

When the temperature of the sea is much lower than that of the air above it, the underwater parts of the hold would be cooled by contact with the sea. Hence sweat would form on the steel parts of the hold below the waterline. This normally would flow into the bilges. The use of dunnage to keep the cargo off the steel parts of the hold and also to keep it a few centimeters above the bottom of the hold would suffice. Ventilation would also help, in reducing the relative humidity in the hold.