Imitating the effect of the old Terragen workd *.wpg files.

[Anthony Appleyard]

In the old Terragen (now Terragen Classic), the user could set up a Terragen world file, which set up the sky and atmosphere and water of the scenario. Thus I soon made a Terragen world file called underwater.tgw , which had water sand-colored so that it looked like seabed sand, and atmosphere with a dense cyan haze like the result of underwater low visibility. How could such a useful scene-setter be imitated in Terragen 3?

[Dune]

Same way. You can use a low cloud without any input, manipulate its density and color. Have you tried searching (underwater) for setups in older posts? I'm pretty sure there will be some tgd's or tgc's. You can also use an inverted (no shadow) sphere slighly higher (just copy sizes and raise by the offset in a otherwise empty surface shader)  than the planet with waves to simulate waves from below.