It is imperative that water is delivered clean to urban centers and towns despite its channelling through waterways, ponds and city aqueducts. Waterways are occasionally polluted by micro-organisms, landslides, pesticides, as well as by human activity and waste. In coordinated efforts to address such problems, water authorities and local governments resort to cleaning facilities whose main task is to filter the water in a timely fashion. In this regard, authorities must be forewarned of imminent or developing pollution issues, so that immediate corrective action can be taken. The installation of sensors that continuously monitor the quality of water passing through specific points in waterways, proves to be an effective way to implement legislative mandates for clean water. We use submerged sensors to gather measurements that can help characterize the quality of water in canals using parameters including temperature, conductivity, turbidity, PH, and pressure. Raw sensor-generated measurements turn out to be of limited help when it comes to monitor the overall water quality and by themselves, can be even misleading occasionally. In this paper, we discuss the main features of Tethys, a real-time water quality monitoring tool, whose aim is to help the Athens water authorities in their ongoing assessment of water quality. Tethys receives as input streams of measurements from stations in the field, detects unexpected events, visualizes the flow of information, and automatically alerts supervisors about potential dangers appearing in waterways. We outline our design choices, filtering mechanisms, and implementation effort in realizing Tethys and demonstrate its real-time use.