Okay wow I'm almost 'caught up' with the series (just finished watching ep9), and damn, this anime could really blow itself out of the water with all the apocalypse stuff. It could take a very depressing/sobering turn.. which would be a huge contrast to all the progress that was made between the kids and their friendships. On the other hand it could still go for a happy ending, like some deus ex machina resolution, where everything turns out okay.. Either way it will be interesting to see how it's handled.
Hmm from the looks of it, the apocalypse will be an ice age. Haha, which is basically reverse global warming. The anime hasn't done too much with those environmental themes, but I guess it's still interesting to keep in mind.
It's still mysterious exactly what role Uroko-sama and the "sea god" plays in this whole thing. There was that little bit with Uroko trying to freeze Akari and Hikari from leaving for the surface, maybe they have some control over the weather? On one hand it just sounds like they want their species to survive, without much of a care for surface people. Which I guess makes sense. Heh we also get the classic "choose how to spend the last days before the apocalypse" dilemma. Land people don't have a choice because they can't go underwater where it is safer, but sea people do!
If this ice age really happens and land people get wiped out, it will be like resetting the ecological situation. Since (at least in the anime's mythology) people used to all live in the sea, and then moved on land.