Sadly a commercial flop.

Then why did the Chinese copy it and are continuing to develop it?

If the technology really does not work, E.ON should not sit on the patent and block saving fossil fuel, but they should give up that patent. Withholding such a climate-saving patent from being used is a crime against humanity, because climate change kills people.

Didn't generate enough, cost a lot, and maintenance was a nightmare too.

Citation needed. Also, wind turbine technology took over 30 years, from 1970 to 2000, to arrive at megawatt scale. The experimental plant at my university had 15 kilowatt in 1991. The Pelamis technology has developed much quicker to half a Megawatt scale.

Sea water and conditions destroy basically everything, it is rather phenomenal.

Maritime technology is difficult because of the salt water, yes. Still, sea ships and offshore oil rigs do exist and we have learned lot from that.

Underwater tidal turbines showing potential, if we do want to harness sea power.

They can only operate near shore at suitable locations. The Pelamis plants can operate away from the shore at the large ocean-exposed coasts of Great Britain, Scotland, Ireland, Iceland, Greenland, California, Mexico, Chile, Argentinia, Australia and Tasmania, South Africa, and so on - which all have a lot of waves. In winter in the North Sea, waves are often six meters high.

The difficulty is to avoid that the large waves destroy the energy converters, and Pelamis apparently solved part of that.