The shapes of the buoys have changed dramatically from Ceto 3...

The shapes of the buoys have changed dramatically from Ceto 3 through to Ceto 6. Universities in Ireland and the U.K. have been working on designing a buoy shape that can capture the maximum amount of energy.

The most notable change has been an increasing in buoy diameter and a reduction in buoy height.

This study looked at various wave types and buoy shapes to gauge performance. Given that this study was completed over two years ago, we could assume that much progress has been made since then.

I can remember reading that the buoyant actuator has just the minimum amount of positive buoyancy to keep it at about two metres below the surface, which to me would mean that upward force is not the only energy producing element.

What ashic1 says about downward force, might in fact be the most dominant element.

From my own experience in the surf, the downward pressure of a large wave above, would seem to be far greater than the upward force of my body with airfilled lungs wanting to rise to the surface.

"The influence of shape and PTO settings on the performance of the CETO 6 is thoroughly studied under regular and irregular waves. The performance of the CETO 6 model under operational sea states is measured and the extreme loads under high sea states are accurately measured. In addition, the influence of combined wave and currents on the performance and loads of the CETO 6 is studied and the performance is compared under long crested and short crested waves. "

http://www.marinet.eu/public/docs/CETO_Hydrodynamics_UEDIN.pdf