Any long exposure can blur the roughness to create silky effect.
The following waterfall was taken on a sunny day. In order not to over – expose the image with 2 second exposure time, I selected very low aperture, ~ f25. Due to very low aperture the depth of field is large so that besides waterfall, all its surroundings are well focused. On the final photo the waterfall looks smooth as silky falling from sky. One of my friends told me that it is just like a painting.
The following sunset on Pacific Ocean was taken at Santa Ana. The ocean was fairly rough on that night. In order to achieve a large depth of field together with smooth ocean surface, I chose aperture ~f15 and 10 seconds exposure time. I was happy with the results that rough water turned to be really peaceful on the photo. To my surprise those people on the beach disappeared on the image.
It is fun to take photos using long exposure, however I have to carry around a heavy accessory with me – tripod.