It doesn't matter that much...at sea, there's nearly always a breeze - or stronger - which will quickly spread the smoke from a relatively small sub-diesel.
Certainly when there are waves, which have an additional disturbing effect on aircurrents at surface-level.
Imagine how an albatros sailes the wind over the waves...smoke from a sub-diesel (emerging at nearly water-level) is more likely to be drawn downwards between two waves...and as soon as it pops over a wave, it's being grabbed by the wind, and dispersed.
In real-life, a watch-crew would probably be earlier alarmed by the smell, then by the sight of diesel-smoke...if the wind is right...and the source is close.
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