What is the ruling on striking the face with water?

If the questioner means washing the face with water during ablution by slapping it, then this is not permissible, because what is required is washing, not slapping. As for slapping another person with water, if it causes harm, it is not permissible. However, if it is a light slap, like sprinkling or spraying, it is permissible for the purpose of alerting the person to prayer, as the Prophet, peace and blessings be upon him, said: "May Allah have mercy on a man who gets up at night, prays, and wakes his wife; if she refuses, he sprinkles water on her face. And may Allah have mercy on a woman who gets up at night, prays, and wakes her husband; if he refuses, she sprinkles water on his face." This indicates the permissibility and desirability of compelling someone towards good.