In this work we address the problem of precise positioning and trajectory tracking for an Intervention Autonomous Underwater Vehicle (I-AUV). Two solutions, based on the Variable Structure Systems (VSS) theory, are proposed with focus on the Super Twisting Sliding Mode methodology. We employ two different sliding mode approaches in comparison with a commonly used double loop architecture PID approach. The tuning of the controllers was carried out starting from the knowledge of the I-AUV real parameters. The performance of the proposed algorithms was verified via simulations in presence of external disturbances on a 6-DoF industrial simulator provided by Saipem.