The first thing to notice about river water is that it is much calmer than the sea. That seems like a common sense statement, but it affects how you treat the waves, which on a calm river are really just ripples. But more subtle than that is the realization that in a river there is no sea swell, so the ripples are more uniform, more balanced and don't have the harmonics in the waves that you see on the ocean. This makes river water look more peaceful. And river ripples generally have softer crests and troughs, especially on a calm day (on rivers you usually have less wind than on the ocean).
Even the bow wake curling away from the bow of the steamer is much more uniform than it would be out on the ocean. Notice how I had to be very careful to ensure that colour changes in the reflection are exactly below the colour changes on the steamer that they are reflecting. The stem of the steamer rakes back from the waterline up to the deck; this same angle appears in the reflection below. This is especially important in calm river water, where the waves are more uniform. Even on a calm sea, the wavelets are a little more random and so reflections are not as uniform and reflect the different facets of ocean wavelets.
The stern wake of the steamer is much more even and appears flatter in a river setting. Notice how, as the ship has moved through the water, the wavelets are disturbed and less uniform and so the reflections at the stern are not as precise as they are in the undisturbed water by the bow.
While I enjoy painting rough sea, calm water can actually be just as challenging as calm ripples demand very close attention to detail.