You can place water ontop of a "lower slab", but not on the top of a double slab. Water also seems to successfully flow into a "lower slab" occupied place, but immediately overflows and spreads out afterward. You can also apparently place water 'inside' a "top half slab", yet not on the top of it. Also if you then place a lower slab next to the water you put on a lower slab, the side facing the new slab of the water completely disappears