Polyurethanes with water expansion properties are manufactured by incorporating polyurethane resins and superabsorbent polymers. This study aimed to present an optimal mixing ratio of a superabsorbent polymer that can express the effective waterproofing performance of a polyurethane material with water expansion properties. The evaluation results confirmed that the waterproofing performance was exerted at a superabsorbent polymer mixing ratio of more than 10% and an expansion ratio of 150% or more, which were then set as the minimum standard value for ensuring waterproofing performance. In addition, chemical erosion (by hydrochloric acid, sulfuric acid, nitric acid, alkali, and sodium chloride), freeze-thaw, and wet-dry cycles were set as deterioration conditions to determine the optimal mixing ratio for the superabsorbent polymer under conditions of use, and the maximum expansion ratio was evaluated after pretreatment under the deterioration conditions. It was found that, based on the minimum standard value for the waterproofing performance set above, the superabsorbent polymer mixing ratio that can ensure an expansion ratio of 150% or more under all deterioration conditions was 14%.