The Biblical account specifically mentions fissuring of “the fountains of the great deep.” There is a possibility that much of the water of the flood was a temporary, forced extrusion of subterranean water. In any event, owing to tidal distributions and the probability of unusual seismic activity, it would simply not be necessary for the entire globe to be uniformly, statically immersed just to set one boat relatively high on one mountain.
I’m not going to leap into my Bible, but Noah sent out several doves in calm seas searching for land, did he not?