A simple, effective and inexpensive water-quality measurement system comprises a housing suitable for travel on or in a body of water; one or more sensors for measuring water properties as the housing travels; a memory for storing information relating to the measured water properties; and software for processing measured properties. This invention combines into a single, inexpensive, and comprehensive system the sensors, computer hardware, signal-processing, Geographic Information System data, and Decision Support Software. This integration yields a system of novel and unparalleled utility. The water properties measured by the system may include: water temperature, water conductivity, pH level, dissolved oxygen, turbidity, water depth, salinity, total dissolved solids (TDS), ORP, chlorides, nitrates, and phosphates. A geographic information system (GIS) based on a global positioning satellite (GPS) technology is operative to determine geographic location and other factors, and an internal or external processor is operative to generate a map coordinating one or more of the listed characteristics to one or more of the water properties. A barometric pressure sensor may also be provided. In the preferred embodiment, the housing is a floating buoy.