Control aspects of a tethered undersea kite (TUSK) energy system are studied. A TUSK system consists of a rigid undersea wing (or kite) attached by a flexible tether to a support structure on the ocean surface or floor. A turbine is mounted on the undersea kite to harness hydrokinetic energy from an ocean current when the kite moves in high-speed, cross-current motions. In this paper, we adopt a six degree of freedom aircraft model that incorporate the translational motion of the kite center of gravity and rotational dynamics of the Euler angles. To simplify the analysis, we assume a fixed support structure, uniform current conditions and a straight-line, inelastic tether. This results in an under-actuated system whose boundedness is addressed with the Lyapunov method and comparison principle. A passivity based PD controller is proposed to control the kite trajectory and orientation. A baseline simulation for typical design parameter values in a TUSK system is studied.