Opinion
Firstly, it is not known how salty the oceans were prior to the flood. It is highly likely that they were far less salty than today in which case the flood posed no problem to fish.
Secondly, it is not known if the fish of the time were particularly suited to only salt water or only fresh water. Again, the flood would have posed no problem if they were not able to survive in one or the other.
Thirdly, it is entirely possible that fish survived in areas or pockets of salt or fresh water - even if some of their kind survived, only a couple of each would need to survive to continue the kind.
Fourthly, assuming fish were the same as today, it is known that fresh or salt water fish can survive for periods in water of the opposite type. They are also extremely adaptable.
Overall, there are many plausible possibilities for how fish could have survived the Flood.
If there ever was a great Flood as described in the Book of Genesis, the whole world, land and sea, was covered by several kilometres of water. This has several serious consequences for the survival of fish.
First of all, it seems as if salt-water and fresh water must have mixed in somewhat equal proportions. The ocean waters would have been diluted with so much fresh water that salt-water fish would have died. The fresh water would have been poisoned by so much salt water that fresh-water fish would have died. For these reasons alone, there may have been no fish alive.
Fish have to eat, just like land animals. There are bottom-feeders in relatively shallow waters; there are fish that rely on sea-weed, again in shallow waters; there are fish that find their nutrients in the run-off from rivers: these would have all been exterminated. Sharks and carnivorous fish that feed off other surface fish would die of starvation, as there would be no prey.
In all, the waters of the world would have quickly become a barren desert with no fish.
Great Sheffield Flood happened in 1864.
It's probably a reference to the great flood.
One of the great catastrophes would have been the 40 day flood.
It never really happened. They lie.
The Great Flood, as outlined in various religious texts such as the Bible and the Quran, is said to have lasted for 40 days and 40 nights.
The first mention of rain in the bible , comes with the great flood at Noahs time.
Genesis (chapters 6 to 9)
God directly says in the Bible, that he would never send another flood onto Earth.
Depending on the Bible translation being used, it is called 'floodwaters' or a 'flood of waters' or a 'great flood' or a 'deluge' (Genesis 6:17).
Assuming you're talking about the "great flood" in the book of Genesis in the bible, it rained for 40 days and 40 nights. Which basically means it rained for 40days NON-STOP.
Days and Nights of the Great Flood Days and Nights of the Great Flood
It rained for 40 days & 40 nights, but the flood itself lasted 150 days