It's quite challenging to harness ocean energy - any infrastructure built has to be very rugged to survive salt, waves and bad weather, and as the electricity is mostly needed on land it needs to be transported as well.
Many small-scale powerplants have been created, and even more have been designed, but as far as I know there are no real success stories out there, except for off-shore wind farms. Oh well, I know one small device that can be bought in boat equipment shops - a wave-powered bilge pump to remove the rain water from an open boat.
It would eventually be nice to see an article summarizing all the attempts on harnessing ocean energy.