GPS signals will reflect off water surfaces. Unlike soil moisture and snow depth, which varies most dynamically only on the day that it rains or snows, water levels can vary quite a bit during the day. This adds a bit of complexity for using the GPS Interferometric Reflectometry (GPS-IR) technique to measure tidal variations. On the plus side, if you are using GPS-IR, then you are simultaneously measuring the location of the GPS antenna in the International Terrestrial Reference Frame, which allows you to correct for any ground motion or monument instability.
GPS-IR was first demonstrated at GPS sites at Onsala (Sweden), Friday Harbor (Washington), and Peterson Bay (Alaska). Shown below are some results from the Peterson Bay effort. More details are available from the 2013 IEEE GRSL paper.
Most recently I have been working with Richard Ray (Goddard Space Flight Center) and Simon Williams (Proudman Institute) to analyze a 10-year time series from Friday Harbor, Washington. Although the newer L2C signals are not available over this time period, the less precise L1 data are. As shown here, the GPS water level estimates are very consistent with the tide gauge. The Friday Harbor GPS site -SC02 – is operated by UNAVCO for the EarthScope Plate Boundary Observatory. NOAA operates the tide gauge.