I've been having fun looking through bagginsbill's atmospheric goldmine link (in his first post on this thread, a link to a forum search URL), but noticed that sometimes the results change. So to make sure I don't lose the really interesting threads I've decided to link directly to the main threads for various atmosphere-related topics

Using Poser Atmosphere To Create Underwater Scenes

These are the main threads I found on this topic:

helgard's "Underwater Submarine" - 9 pages. Seems to be the main thread in this forum covering the subject. Also covers getting big waves that look good from above the surface.

AnArdvark's "Water surface from below: shader advice" - 1 page, worth a look

(momodot's "Refraction node question" - 3 page thread, mainly about getting a realistic water-air surface when viewed form the air side)

(image is the underwater one bagginsbill's used in his first post in this thread - just a visual cue for what this post is about!)

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.