The oceans are always moving; not much at times, it may appear, but they always are, either due to waves or the twice-daily tides. The rotation of the Earth and Moon together with the wind make the oceans continuously move.
We can harness this movement in the form of waves or tides. There are many ways to obtain energy by capturing this continuous movement either at the surface or below it.
The video below shows a few examples of how wave energy can be harvested.