I am not a marine biologist who has petted everything in the 7 seas.
I am a tourist interested in local native fauna. Usually, "it tastes like chicken". Or shrimp.
Greetings, shark shin is like sand paper, they say it has less drag in water then a smooth surface. I am not sure if the some thing happens air.
Shark skin is scales. It feels very smooth when rubbing head to tail. It feels like sand paper when rubbing from tail to head.
And Dolphins have a very slippery skin surface and an almost perfect laminar flow.
Dolphins have skin, like you said, not scales. To me it feels like "solidified wet oily greasy fat". It feels like what many people imagine snakes feel like?
Some Whales have tubercles on their fins. My current understanding is they mostly effect the leading edge of the fin by creating vortices, which makes the remainder of the fin more efficient.
It sound like VERY carefully placed and sized divits may be better than smooth on the leading edge.
I don't think 'rough' would make any real difference in efficiency. Whale tubercles look huge to me. Golf ball dimple/tubercles look tiny.
I can not see it being worth the effort after considering water and air properties of density, viscosity, and moving speed.
G-