It is part of the natural physics of water and other fluids to always find their level and remain flat. If disturbed in any way, motion ensues until the flat level is resumed. If dammed up then released, the nature of all liquids is to quickly flood outwards taking the easiest course towards finding its new level.
If the Earth is an extended flat plane, then this fundamental physical property of fluids finding and remaining level is consistent with experience and common sense. If, however, the Earth is a giant sphere tilted on its vertical axis spinning through never-ending space then it follows that truly flat, consistently level surfaces do not exist here! Moreover, if the Earth is spherical then it follows that the surface of all Earth’s water, including the massive oceans, must maintain a certain degree of convexity. But this is contrary to the fundamental physical nature of water to always be and remain level!