Anything that moves in a stream of water or air can give energy:
http://www.futura-sciences.com/magazine ... eau-17448/
like an oscillating leaf (which can have a good return like for a fish or a dolphin), a flag in the wind, an object shaken by the waves, but the difficulty is to convert this movement into electricity with a good return:
see the thesis on the strong magnet converter with all the secrets and real problems:
www.theses.ulaval.ca/2011/27772/27772.pdf
The propellers of current wind turbines are elongated very fine to withstand storms while having good performance in winds or medium water currents.
Other solutions with large surfaces have problems with storms, and therefore may be fragile, as often broken in wave converters!
A much simpler piezoelectric converter, of the gas lighter type, has a good yield only in ultrasound and therefore hardly usable!
We could use the movement to mechanically pump water or pressurized air into a large tank and thus store its energy to use later, but the performance is also to be optimized?