Conventional tide gauges are usually housed along the coast. Satellite altimetry works well in the open ocean but poorly near the coast due to issues such as signal contamination by land returns. These limitations lead to an observational gap in the coastal ocean. Using data collected by a GPS installed on top of an anchored spar-buoy in Tampa Bay, we retrieved water levels through a combination of precise positioning and interferometric reflectometry. Individual water level retrievals agree with a nearby acoustic tide gauge at ~16 cm level. Amplitude and phase of the major tidal constituents are well recovered by the GPS spar-buoy measurements. Over a 2-year period, agreement of daily mean sea levels measured by the GPS spar-buoy and a nearby acoustic tide gauge is 3.1 cm. When sea level data measured by the GPS spar-buoy are included in the coastal ocean circulation model, low-frequency error propagated from the open boundary is reduced.