How could the ocean water NOT mix with the fresh water? And where did all the water go after the flood if it came UP from the ground?
You’re presuming the existence of oceans before the flood. The preflood world likely was comprised of shallow lakes and low hills. The flood itself was cataclysmic. In some areas the land sank. In other areas mountains arose. The floowaters ran off into the depressions, forming the oceans which became salty as minerals were washed from the drying land.