I say, and with God's success: Watermelon juice does not differ from pure water except in taste. If this characteristic appears in it, it becomes pure but not purifying for ritual impurity. It is permissible to remove actual impurity with it, but it is not permissible to perform ablution or bathing with it. Refer to: Maraqi al-Falah, pp. 26-27, and the commentary of al-Tahawi on the Maraqi, pp. 19-20, and God knows best.