The attraction of a tidal generator is that water has 800 times the density of air, so it contains 800 times the energy when in motion. That basic fact explains why tidal generators are attracting the attention of energy researchers. Among the latest developments in this area is a tidal generator that takes the form of something approximating an underwater kite. Developed by a Swedish company called Minesto, the kite assembly, consisting of a wing and turbine, is attached by a tether to a fixed point on the ocean bed.
Hydrodynamic forces on the kite caused by the current velocity makes the kite to move. A control system gives it a trajectory that maximizes power generation. The speed of the kite determines the flow velocity to the turbine. By angling the kite into the current, the flow velocity can be boosted by about ten times the current speed. The electricity is transmitted onshore through a power cable inside the tether. With a wing span of 12 m the kite is designed to run at a speed of 16 m/sec at a current velocity around 1.6 m/sec.
You can get more information about Minesto's kite here, along with a YouTube movie of it: http://www.minesto.com/technology-4/deep-green-technology-37