ok, here's an idea..

How about.. you render the city once with the great amount of myst. and render the ships once with less. then use masks to combine.

If you want to keep more sense of depth and shape within the mysty area's you could also make a grey render of the city without any mist, kinda like a GI-render. Maybe a nice hdri could help for that. Then overlay the gi-render in blender or photoshop or paintshop or whatever u use. The GI-shadows will give extra definition to the shapes in your render which would otherwise be significantly less tactile looking due to the amounts of mist.

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Ray

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