Fins are used to compensate for the poor thermal conductivity of air. Notice that fins are never beyond a certain length on HS. Thats the length at which normal airflow will have removed nearly all the excess heat before it can pass farther down the pin/fin.
With water the thermal conductivity is probably high enough that the thermal conductivity of copper is matched just by the surface area of a Maze-type design. If thats true then pins would be unneeded.
At least that would explain why waterblocks are so different from sinks.
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