Gargantia eh, what makes that thing unique? heard it's as hyped as Attack on Titan.
The story is fairly unique/interesting...kinda out there though. MC and his AI sidekick are pretty unique, I'd say the world is unique...unless you've seen waterworld.
I wouldn't call it unique, personally. The concept has been done with at least once before with Full Metal Panic... war meets slice of life. It's a decent series, that almost fell apart. But the ending saves it quite well.
I don't have any series to recommend (last few seasons have been pretty weak imo) but I will +1 Shinsekai Yori.
That is a unique series. Pacing is fine. Not a deep or complicated story, but it really makes you think.