Water-level monitoring is important for climate analysis, water resource planning, and monitoring of disasters, such as floods, droughts, and storm surges. The technology of global navigation satellite system-reflectometry (GNSS-R) has been demonstrated previously and can be used to retrieve data on various water levels, such as sea, lake, river, and reservoir water levels. In this paper, we focus on the branch of GNSS-R that is based on the conventional geodetic receiver. In order to improve precision and promote the actual application, we present a multi-site multi-signal GNSS-R combination method. Furthermore, to address the phenomenon of “double peaks” in spectrograms of GPS L2P/L2W signal, we present a strategy to retrieve water levels from the L2P/L2W signal captured using Z-tracking method. Data of a GNSS network, which included a base station and three monitoring sites, established in a reservoir of the South-to-North Water Diversion Project were analyzed. The multi-site, multi-signal combination method was used to form a 6-h water level retrieval series and the retrievals showed a root mean square error (RMSE) of 6.0 mm. This is the first example of GNSS-R water-level retrieval achieving a millimeter to centimeter scale precision. The ability of the GNSS technology to simultaneously monitor water level and displacement based on the same GNSS network were also discussed and demonstrated. This study showed that GNSS-R could achieve high precision, high sampling, increased service, and lower cost, thereby representing a crucial progress in the practical application of the GNSS-R technology.