This work focuses on the automatic control of the length of a tether that links an underwater vehicle to the surface, with the objective to prevent the tether from becoming taut or getting entangled due to too much length being deployed. The solution proposed here consists of equipping the tether with a balanced buoy-ballast system that gives the cable a Vshape in the vicinity of the vehicle. This system offers a passive compliance by smoothing the movements of the tether and damping external disturbances. The tether length is adjusted by an active feeder on the surface, whose control relies on the reading of a flex sensor embedded in the V-shape portion of the cable. The experiments conducted on a real ROV in a pool allowed validating this mechatronic compliant-actuated system, which can adapt to the movements of the underwater vehicle while it executes longitudinal and curved trajectories.