I like this. A lot. I have a few questions though: What force(s) brings the water and other material coming off the rimfall back into the interplacial cleft? Is there and opposite-type gravity on the surface pointing 'down' that would allow people to walk around upside-down relative to the upper surface?

Your terminology and world-shape remind me a lot of the Discworld, which is pretty awesome, whether intentional or not. Are all 'planets' in this campaign setting similar in there appearance? Are there perhaps some with water sliding out of their interplacial clefts to run into their pelagic bores and then repeatedly cycled in such a way?