This paper proposes novel cooperative navigation solutions for an Intervention Autonomous Underwater Vehicle (I-AUV) working in tandem with an Autonomous Surface Craft (ASC). The I-AUV is assumed to be moving in the presence of constant unknown ocean currents, and aims to estimate its position relying on measurements of its range to the ASC. Two different scenarios are considered: in the first scenario, the ASC transmits its position and velocity to the I-AUV, and the I-AUV has access to readings of its velocity relative to the water. In the second scenario, the ASC transmits only its position, and the I-AUV has access to measurements of its velocity relative to the ASC. In both cases, it is assumed that an Attitude and Heading Reference System (AHRS) mounted on-board the I-AUV provides measurements of its attitude and angular velocity. A sufficient condition for observability and a method for designing state observers with Globally Asymptotically Stable (GAS) error dynamics are presented for both problems. Finally, simulation results are included and discussed to assess the performance of the proposed solutions in the presence of measurement noise.