A buoy-based GPS receiver and meteorological sensor are used to estimate directly the absolute precipitable water (PW) overlying a coastal ocean site 8 km from shore. During an 11-day experiment, one-second GPS data collected at the buoy and at a shore station are combined with 30-second data from four distant GPS stations to estimate the buoy position, zenith wet delay, phase biases, and receiver and satellite clocks using double-differenced phase processing with ambiguity resolution. GPS-derived PW at the buoy compared to radiosonde measurements (20) and to half-hourly GPS-PW values (384) from the nearby shore station show an rms agreement of ¿1.5 mm and ¿1.8 mm, respectively. Hourly means (170) of the GPS-measured vertical motion of the buoy show a ¿24 mm rms agreement with a NOAA tide gauge, equivalent to about 4 mm of PW. GPS-derived PW from buoys may have the potential to improve weather forecasting, calibration of satellite-based sensors, and climate studies. ¿ 2001 American Geophysical Union