It seems an additional hurdle is anti-fouling. Those barnacles and algae act as an insulator, making the cold-water cooling advantage less and less of an advantage.
>> Those barnacles and algae act as an insulator Maybe not. Barnacles probably conduct heat at a similar rate to water. And they create a rougher surface with greater contact area to the surrounding water. And some of them actively filter water, push it around. Perhaps having a layer of barnacles woudl increase cooling.
Barnacles and other accretive life forms means that the thermal gradient drops sharply, and hence the heat flow rate drops significantly.
Take a comparison is between (say) 25C heatsink in contact with 10C water across a boundary layer that likely on the order of 10mm thick, versus what is effectively a static fluid ~50mm thick followed by the same boundary layer.
So the unmixed fluid is going to be ~6x [1] thicker, for a heat flow rate 1/6th the original. And that's assuming that barnacles do in fact conduct as well as water, something that's unlikely (water is ~0.6, human flesh ranges from ~0.2 to ~0.5)
[1] Insert a bunch of unstated assumption about the fluid flow rate, salinity, etc etc.