Waves have several causes. Probably the most common is wind. But a change in density within the water (in other words, one area of water that's more dense and heavier moving next to an area of less dense, lighter water) can get a wave going. Other causes include a small disturbance within or below the water (like the movement of a ship, submarine, or whale), earthquakes (which are the main causes of tsunamis) and the gravitational pull of the sun and moon.
---D.M.
