This research aims to develop a tide and wave monitoring system, which can be mounted on the data buoy, for monitoring the water surface elevations at the river mouth region. Because wave data can be calculated from the raw data of GPS receiver and the gyroscope, this research also intends to compare the wave data from both sensors. A system including a data logger, GPRS modems, a RTK (Real Time Kinematic) GPS receiver and a gyroscope has been integrated and mounted on the Suao data buoy, which is operated by the Coastal Ocean Monitoring Center (COMC), National Cheng Kung University (NCKU), Taiwan, to provide the wind, wave, and current data at the coastal area near Suao. The GPS-buoy system measures the water surface elevation, the velocities and the altitude of the buoy hourly. These raw data are then analyzed to obtain the wave and tide data. The tide data obtained by the GPS-buoy are validated by comparing with those obtained from the neighboring Suao tide station. Inside the GPS-buoy, water surface elevation obtained from the gyroscope have also been compared with those obtained from the GPS receiver. This study shows that the tide data obtained from the GPS-buoy agree well with those recorded by the neighboring tide station. Furthermore, the water surface elevations measured directly by the GPS system agree well with those determined from the raw data of the gyroscope on the same GPS system.