An exemplary inventive freeboard detection system includes a tubular watertight temperature-sensing device and a computer. The temperature-sensing device includes a printed circuit board assembly (PCBA), a potting compound, and a hollow rigid tube. Inside the tube the potting compound encapsulates the PCBA, which includes a printed circuit board (PCB) and multiple temperature sensors closely and equidistantly arrayed along the length of the tube. The temperature-sensing device is vertically secured in a partially submerged state to a vessel with the expectation that some of the vertically arrayed temperature sensors will sense air temperature and others will sense water temperature. On an ongoing basis, the computer receives signals from the temperature sensors and processes the signals to monitor freeboard values, which the computer calculates based on differences in temperature measurements corresponding to pairs of consecutive (and/or nonconsecutive) temperature sensors. A maximum calculated difference in temperature measurements is indicative of the location of the water surface.