Humanity discovers a wormhole on the edge of the Solar System. They send a probe through and discover there's a habitable waterworld on the far side. A colony program is developed, and the first of many ships is launched to settle Poseidon.
Unfortunately, a blight strikes Earth after the first colony ship departs. Civilisation nearly collapses as wars are fought over the few available food supplies. A new order arises, balanced between the surviving nation states, the megacorporations, and the Global Ecology Organisation. After a century or so, the situation has stabilised enough for a mission to be sent to Poseidon, where they discover the colony has survived. The descendants of the original colonists have abandoned technology (they had to assume that they were the only survivors of humanity, and so dropped to a sustainable level of technology).
With Earth still rebuilding, Poseidon would be irrelevant if it were not for the recent discovery of a strange substance called xenosilicate or 'Long John'. It's a naturally occuring substance that enables incredible genetic manipulation - including artificial life extension. Poseidon becomes the new frontier as the Long John Rush draws the megacorporations and millions of new colonists to the waterworld. The 'natives' - the original colonists - are none too happy about this flood of invaders.
So, you've got Dune at sea, mixed with elements of cyberpunk and westerns. Add in uplifted dolphins and orca, lots of intrigue and strife, and the true origins of xenosilicate...