The modeling and control of tethered undersea kites for power generation are discussed in this paper. In the first part, we apply the added mass model to underwater kite systems. In the second part, the stability of undersea kite systems is analyzed using two frameworks. The dissipative property of the hydrodynamic force is provided first. Based on this property, the input to state stability of the undersea kite dynamics is established. Moreover, based on the input to state stability, the ultimate boundedness of constrained undersea kite dynamics is also discussed. Finally, an input–output system is proposed, and a passivity-based control signal is designed. The proposed control signal is tested in a baseline simulation, which yields consecutive figure eight cycles with positive net power generation.