I use a bunch of divs here to simulate a canvas, but the principle is the same. Every 40 game loops, each water tile will have its lightness value change to a random number within a range. This gives an impression that it's moving. You can manipulate the speed of the body of water by changing the range: wider range, higher speed. Because of this, there will likely be two water tiles: river and lakes/oceans, each with a different speed.