Why paddy plants do not experience water logging despite being immersed?

Its all down to the paddy plants individual adjustments to its environment. By means of osmosis, it selectively allows just the right amount of water in through its cells (ie via the roots) and no more. In other words, its cells are selective in how much water it will take -it needs some of course to feed on and keep stiff & upright- and when it has enough its osmosis level is at a state of equilibrium, so will not accept any more.