This work explores both modeling and control of the experimental Ciscrea autonomous underwater vehicle. A 6-degree-of-freedom model is presented and validated for turn and emerge/sink maneuvers. Then, a constraint compensating algorithm is proposed based on quasi-sliding mode conditioning ideas and added to a pre-existing inaccessible proportional-derivative controller in order to improve the overall closed-loop response. By considering actuator constraints, the employed technique allows path following at greater speed than the original controller for a given error tolerance. Experimental results on the so-called Ciscrea underwater robot are presented.