Ah, the great thing about a mechanical fuel pump - you know that the black smoke is meant to be there ;)
But more seriously, it's interesting that the smoke is on cold start and not also warm start and that the engine is in proper use. Presumably you can easily rule out a number of usual factors that may be possibilities on a mechanical engine. I wonder where the boat is?
Similar to what Bill said, I wonder if the computer has a special setting for cold start, in effect the combination of injection timing, number of injections per stroke and fuel amount (plus glow plug if relevant) and that in a warm climate it isn't necessary and produces too much fuel for just a moment. Just a guess.
Separately, I am more than happy to be corrected, but I thought black smoke is inefficiently burnt fuel. Unburnt fuel produces white smoke. I've had an engine with a hole in the valve leading to no compression in one cylinder and hence no combustion and lots of white smoke. The cylinder at fault was easy to identify by disconnecting the fuel line to each injector one at a time until the smoke stopped when no fuel was going to the cylinder at fault. Sure enough, when the head was taken off there was a hole about 2 mm across at the edge of a valve.
John, Popeye, Maramu #91