We present the design and field test of a rugged FBG sensor prototype for high-sensitivity measurement of underground water level. Pressure sensors have many fields of application, ranging from environmental monitoring to the oil and gas industry. In particular, pressure sensors can be used to monitor the stability of dikes and embankments by measuring the inner phreatic level at their foot to detect anomalous filtration and excess of pore pressures. For this application, rather high sensitivity at an affordable cost is required. Fiber optic pressure sensors have been explored with different solutions, but the technologies proposed so far have either small sensitivity, and hence are befitted for large pressure ranges, or are based on interferometry, and hence require rather expensive laser sources. The sensor described in this paper exploits a 3D-printed mechanical transducer to convert external pressure in longitudinal strain along the fiber. A second FBG, embedded in the sensor, is used to compensate for temperature cross-sensitivity. The structure is enclosed in an aluminum alloy case to withstand harsh environments and installation procedures. Pressure and temperature sensitivities of the sensor are about 20 pm/cm H2O and 17 pm/°C respectively. Three sensors of this kind have been successfully tested in a large scale dike at the Flood Proof Holland facility, in Delft, Netherlands.