water-core

from The Century Dictionary.

• noun In founding, a hollow core placed inside the mold, within which a current of cold water can be made to pass to absorb the heat and hasten the cooling of the casting: used especially to cool the bore of cast guns.
• noun In some forms of car-axle, a quantity of water in a hermetically closed cavity, intended to take up heat from the journals.
• noun A blemish, common in some varieties of the apple, in which the flesh about the core assumes a watery, translucent appearance.