Control of kite trajectories for a tethered undersea kite (TUSK) energy system is studied. A TUSK system consists of a rigid undersea wing (or kite) attached by a tether to a buoy on the ocean surface. 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. We adopt a six degree of freedom aircraft model that incorporates the translational motion of the kite center of gravity and rotational dynamics of the Euler angles. We assume TUSK system is connected to the fixed surface buoy with a straight-line inelastic tether and harnesses energy in a uniform current. Improved models are developed for added mass effects and hydrodynamic moments due to buoyancy on the underwater kite. We apply a passivity based controller, which has been previously developed for airborne wind energy (AWE) systems, to control the underwater kite trajectory and orientation. A baseline simulation for typical design parameters values in a TUSK system is studied.