Well, yes, I suppose, but you still don't get something for nothing. Wave motion generators have been around for a while, and pendulum inertial generators aren't new technology either. Ultimately you have to consider in making this a man-portable device (such as in your backpack), is the dynamic weight load swinging around in your pack all day any better than, say, having a handcrank device that generates as many joules? The inertial motion of the pendulum induces additional drag/load in your pack, which at a level sufficient to make LEDs flash off an on probably need not be that noticable. You could do the same with a faraday generator; those LED flashlights that you shake so the magnet passes back and forth through the coil.
The best wave generators I came across used a bi-directional impellor system so that as the crest passed by, the force pushed the impellor by directing the flow of the water through one set of one way gates, then the trough pulled water back through a different set.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)