This paper presents a novel framework for a robust motion control scheme of an eight-thruster underwater vehicle. It combines a model-free approach intertwined with a model-based approach such as sliding mode control (SMC) to counter the unknown disturbances and decouple them to provide better tracking. A proportional-integral (PI) control-like structure is taken as the first sliding surface. A proportional derivative (PD) control-like structure is proposed as the second sliding surface for trajectory tracking. This motion control works for any fully actuated or over-actuated vehicle. Initially, the dynamic model and vehicle configuration are presented. Then the vehicle’s closed-loop behavior is studied in the presence of underwater currents. Later, the study considers external disturbances and compensates them with the help of a nonlinear disturbance observer. Lyapunov’s direct method and Barbalat’s lemma ensure the asymptotic convergence of tracking errors. The proposed controller performance is evaluated using a detailed comparison study with different model-free and model-based controllers from the literature. Later, the control scheme’s effectiveness is demonstrated numerically with the help of computer-based simulations. The robustness against the parameter uncertainties, underwater currents, and unknown disturbances is also presented. © 2023, The Author(s), under exclusive licence to Springer Nature B.V.