A method of controlling a tethered, underwater, water current-driven turbine, power-generating device. The device is comprised of dual turbines and dual rotor blades turned by fluid flow, each turbine including one or more generators driven by rotor blades. The device is connected by device tethers to a strut, which is moveable to control depth of the device. The strut is connected to the ocean floor by tripodal tethers: a main tether, a left side tether and a right side tether, which are strut control tethers. One or more winches are controlled to maintain operation of the device within set parameters by varying the tension on one or more of the strut control tethers.