For reasons that will soon be apparent, a typical preflood sea probably had a small salt content, as if you mixed two parts of fresh water with one part of seawater. Living things have many marvelous, semipermeable membranes that allow some liquids or gases to pass through, but not others. For example, capillary walls are semipermeable membranes. Oxygen in our lungs can pass through capillary walls and mix with our blood, but blood does not normally pass through those walls. This is called osmosis .
Fish have a water problem.
Freshwater fish have greater salinity in their blood than is in the water they swim in, so water seeps into their blood by osmosis. To correct this problem, freshwater fish seldom drink, and their kidneys secrete a watery urine. Conversely, saltwater fish have less salinity in their blood than is in their saline environment, so osmosis forces water from their bodies. Their kidneys pump out so little water that saltwater fish seldom urinate. During the flood, fish would have tried to stay in the most comfortable regions of the volume of water that was their preflood habitat.