<p>Monitoring coastal sea levels is crucial for assessing the impacts of climate change and predicting flood risks. In this context, GNSS-Multipath Reflectometry (GNSS-MR) has emerged as a promising technique, offering an efficient and stable method for sea level retrieval. To fully realize the ability of various GNSS observations in GNSS-MR for the water level inversion, the GMR-Water, an open-source MATLAB-based software, has been developed which is capable of efficiently using the multi-observations including Signal-to-Noise Ratio (SNR), carrier phase, and pseudo range. Moreover, the GMR-Water supports the integration of multi-GNSS, multi-frequency, and multi-observation data, employing advanced error correction and combination algorithms to ensure high-precision retrieval of water levels. The software is designed with a user-friendly interface, equipped with batch processing capabilities. and it offers a comprehensive set of visualization tools to enhance the interpretation of results. We validated the performance of GMR-Water using observational data from the AT01 GNSS station, where the software was used to process the GNSS observation and analyze the water level measurement. The results demonstrate that GMR-Water can effectively retrieve water levels with high accuracy, achieving consistent and reliable results across various GNSS observation types. The performance of GMR-Water proves its potential as a valuable tool for precise coastal sea level monitoring, contributing to the further development and application of GNSS-MR water level monitoring technology.</p>