Watercore is something most folks see as a disease but really it not in the classic sense, in that it is not pathogen induced.  Often you do not notice anything from the outside of the fruit, but when cut into you will notice a transluscent area instead of the opaque white or yellowish color normally present in apple flesh.  Really an affliction of poor management, but with some varieties being far more prone than others. The above photo is an extreme example, most affected apples show only a small degree of damage. It is usually a  sign of boron/calcium imbalance. It isn’t unhealthy, and often does not affect the flavor adversely either. The sugars are concentrated in the area, and in fact such apples are sometimes sought after in many cultures. More often it is considered less than desirable, especially from a marketing standpoint. The watercored area can also be high in sorbitol, a non digestible sugar, making it sweet but lower in calories. It is also non-digestible to yeast, and so fruit with high watercore incidence can leave a nice residual sweetness in hard ciders and perries.