Raw potatoes contain 79% water. As one would expect, the water content of boiled potatoes is similar to raw potatoes at 77%. Cooking in a dry environment reduces moisture content but not always as much as one would expect. For example, the water content of baked potatoes is 75% and that of microwaved potatoes is 72%. Frying causes the highest level of water loss, resulting in a 39% water loss in French fries to as little as a 2% water loss in potato chips.

Source: National Library of Medicine