The boat is moving at the same speed in all of these photos, 50 cm/s. According to Kelvin's theory, all three of these wakes should look the same, but they don't. Try to count the transverse waves behind the boat (the little white spot at the top of each image). Left: Skewed waves. Here, the surface is not moving, but there's a current under the surface. Centre: Same speed, also with the surface at rest, but for this case there's an underwater current against the direction of motion. Right: For this case, the boat and the underwater current are moving in the same direction, still with no surface motion. (This is shortly after the boat started moving, so you can see that the waves are closer together at the back).