Catastrophic Plate tectonics give a mechanism for the deepening of the oceans and the rising of mountains at the end of the flood.
As the new ocean floors cooled, they would have become denser and sunk, allowing water to flow off the continents. Movement of the water off the continents and into the oceans would have weighed down the ocean floor and lightened the continents, resulting in the further sinking of the ocean floor, as well as upward movement of the continents.1 The deepening of the ocean basins and the rising of the continents would have resulted in more water running off the land.
The collision of the tectonic plates would have pushed up mountain ranges also, especially towards the end of the Flood.
Batten D., Ham K., Sarfati J., Wieland C., The Answer Book Pg 164
1. The geological principle involved is isostasy, where the plates are ‘floating’ on the mantle. The ocean basins are composed of denser rock than the continents, so the ocean basins sit lower in the mantle than the less dense continents with their mountains.