See the horizon in the image, I can't seem to get it to be anything other than razor-sharp. Increasing & decreasing fog & haze has no effect. Does anyone have any nice tips about this?

V5I? If so, you could probably use the 'Distance to camera' input to control the colour of the sea material, so that it fades towards the horizon colour at extreme distances.

Yep V5I. Thanks for the tip, but I'd rather have some way of fading the visibility of the land plane. I got a volumetric atmosphere to work, but I lose sky detail (I know, I haven't added the clouds yet ;)) and increase the render time dramatically (I make animations so any increase is compounded by the number of frames).

You should be able to delete the infinate ground plane to get rid of that effect, I would suggest it anyway as I usually delete it so I can control my horizon line myself with terrains. Anyway, if it's possible for you to do so and get away with it I would recommend that approach.

I like the example you did with the volumetric atmosphere but i understand what you mean about render times. It looks like you have not set Fog in Sky. With a standard atmosphere make sure you have the Fog in Sky Thickness set to 100% and max altitude around 20-30. Then set the fog to around 200 with around 20-30% falloff and gather at low altitudes. Tweak max altitude and falloff to get the exact look you want. This is in V5 Esprit, i guess it's same/similar in Infinite?

Bingo - a sticky bun goes to CD :D Fog In Sky indeed fixed it. Funny how you can not see stuff, but when pointed out to you seems totally obvious :D Thanks for the tip ;) Now I just need to fix it again after tweaking it for volumetrics. I seem to have lost some of the nice yellowness of the close trees.