Many thanks everyone, yes some good ideas to pick over here, I'll chew on the power and pressure ideas. The hydro argument over potential I like, guess if there was no turbine in the way, the ejected water would be at a greater velocity, all (or nearly all) the PE being converted to KE. I generally feel happy now about the hydro idea but will have to think on about the wind generators. Thanks
As an aside, I was told some time ago that the tidal barrage scheme in North France would result in the slowing of the Earth's rotation by some 0.25 seconds in 1 million years (I haven't actually bothered to do the calculation myself) - presumably as we move more and more to 'renewables' such as wind generators, we'll slow down the rotation a bit more - after all we don't get energy for nothing, it has to come from somewhere.