Make the bottom of the world the top of the world, the inner layer the outer layer.
That is, make it a large planetoid that has a rapid spin. The spin is great enough to force everyone towards the outside. The oceans would be on the surface, and here comes the handwaving. The gravity of the planet is strong enough to keep the water on the surface from being flung off by the centripetal force. Not sure if that is at all possible, but it is a good plot device. Maybe the 'water' has a very high surface tension that keeps the 'water' wrapped around the planet. The water is 'sticky' enough to adhere to the planet surface.
As the residents moved deeper underground, towards the center of the planet, the 'gravity' would actually lessen, and the 'real' gravity increase. If they go deep enough, the 'gravity' would reverse, so 'regular' gravity would pull them towards the center of the planet.
When they got to the 'bottom' (the top layer) of the cave, there would be openings in the crust layer, so they would be looking at the bottom of the ocean. Again, here is where handwaving comes in. The centripetal force would keep the water from 'falling' into the cave, being 'flung out', and the surface tension/stickiness would cause it to film over the hole. If you jumped in the water, you could swim to the 'bottom' which would actually be the surface, and you risk being cast into space by the centripetal force. The opposite of 'diving into' the water, you would be 'diving out' of the water. At this point, how far up you go depends on the actual gravity and escape velocity of the planet. That is, you are flung out by the spinning, and then brought back by gravity.
Everything would be a handwaved balance of forces between 'normal planetary mass gravity' and the 'centripetal force artificial gravity' of the planet spinning, adjusted as the plot demanded.
A certain suspension of disbelief would be necessary, but that is what fantasy is all about. Magic if necessary but not necessarily magic.
The night-day cycle could be altered from the planet spin by pointing the axis of spin towards the sun, and the 'night/day'dictated by the revolving of the planet around the sun in some weird planetary orbital configuration. That is, at the equator, the stars would be zooming across the water but the sunshine would maintain a constant brightness, at the poles they would see either space or the sun as the planet circled around the sun.
It would be fantasy weirdness par excellance.