A submersible turbine-generator unit includes two or more turbines mounted on a common shaft with a generator therebetween. Each such turbine is capable of providing relatively low-speed, unidirectional rotation under a reversible ultra low head pressure and/or low velocity fluid flow. A plurality of airfoil-shaped turbine blades mount parallel to the common shaft and transversely to the direction of fluid flow for rotation in a plane parallel to the fluid flow. The turbines convert energy in the waterway currents into mechanical energy that will transfer through the turbine shaft to a cylindrical permanent magnet generator which will convert mechanical energy in the form of RPM and torque into electricity. Electricity from the variable speed electric generator is controlled, synchronized, conditioned and transformed into utility-grade electricity using a power electronics system in each module.