if (cm > 200) sendAlert();
Change this to your desired water level (i.e. the distance between the sensor and the water level - the higher the number the lower the water level), and in the sendAlert function you send out the alert in whatever way you like to.
You will want some way to prevent alerts being sent time and again - your 5-hour delay() is a very crude (but in this application effective) way of doing this.