This study investigates the vertical plane motion control of underwater gliders operating in complex marine environments characterized by significant external disturbances. To address this challenge, a composite pitch motion controller that integrates an extended disturbance observer (EDO) with nonlinear model predictive control (NMPC) is proposed. Considering the constrained onboard computational resources typical of underwater gliders, the implementation adopts an offline optimization strategy for real-time control feasibility. For achieving high-accuracy disturbance estimation and compensation, an EDO is developed on the basis of high-order disturbance dynamics reconstruction. Simulation results validate the enhanced robustness and disturbance rejection performance of the proposed method.