In this paper, the path following problem of an underactuated unmanned surface vehicle (USV) in the presence of environmental disturbances is considered. To compensate for the sideslip induced by drift forces, an adaptive Kalman-like observer is proposed to estimate both the path following errors and the unknown sideslip angle. Based on the estimated angle, an adaptive observer-based line-of-sight (AO-LOS) guidance law is then constructed to ensure convergence to the desired path. It has been proven that the proposed sideslip estimator is uniformly globally exponentially stable (UGES), while the equilibrium of the cascade system, composed of the sideslip estimator and guidance law, is shown to be uniformly semiglobally exponentially stable (USGES). Several numerical and experimental examples are provided to validate the theoretical results and demonstrate the practical applicability of the AO-LOS guidance law.