One would think that water's density is smaller than the rock. Why wouldn't it eventually rise to the surface above the denser material if the surrounding rock wasn't solid?
Nevertheless, I have no idea of the porosity level that could exists at those depths.
2) At 700km depth pressures are of order 21 Giga Pascals (~207000 atmospheres) and temperatures about ~1900ºC. Water is not the water we know at these conditions, it is a supercritical fluid. The line separating liquid and gas phases ended at much lower temperatures.