Ok. There is a heat layer over the metal that is the radiated heat. The barrier is say... 1-2 mm. If you have water moving over this barrier, you are carrying the heat away. If you have a bumpy surface, say a sand blasting where the surface is not 1mm varience, then your heat barrier is not effected and you could call the surface change cosmetic. If the surface deformity extends in a significant way to bring this heat barrier out into a faster flow of the water, or increase surface to water heat transfer area, then you have an advantage. I look at hara's dimple block, jaydee's dimples in his avatar, or the microchannel. On those examples you have a large enough deformity to add to heat->water exchange. The sandpaper approach or sand blasting, doesn't do jack from my understanding.
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