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* 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 or something else.
*soon only to be available in two flavours -  DAZ and Renderosity