Wow, connecting Abyss and Avatar - would be interesting. But unlikely. I guess that JC is looking for more inspiration for weird life forms and they are abundant in the deep sea. He used shallow surface marine life to create the land plants and animals on Pandora and make them as fantastic and strange as they are. I guess the underwater world might be inspired by Earths deep sea creatures mixed with many other elements of course. I dont think there will be a deep sea exploration on AV2. If there should be, then I think the only option would be that the skypeople return and for some reason go down there. Underwater mining? Maybe. But that would be sort of a repeat of AV1 wit humans making a base and then having to deal with the sea life that attacks them.
My guess is that not more than a few percent of the movie will take place underwater, because that woul dbe too one sided.

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