I've been scratching my head and thinking about tidal power, especially as we are considering a property on a tidal island in western Europe, where the tidal reach is the second highest in the world.
Designs based on a Sandia-Savonius rotor immersed in water and with the generator at the top of the shaft seem to have been successful for the SeaPower EXIM project. A Savonius tidal design would need a modified version of the "brake disc" generator, possibly with more magnets to get decent voltages and frequencies at the 50-100RPM that is going to be produced.
There was some discussion here about tidal power back in May. I guess the big problem is the environment, in particular fouling and corrosion. GRP might be a material to consider, or maybe stainless steel. At least the generator can be out of the water, though, since the Savonius is a VAWT design.