At first glance this question would seem to be the ultimate “no-brainer”. A typical response might be:
“It was a flood! That means lots of water. Marine species live in water.”
But, of course, there are different types of aquatic species. Primarily there are freshwater fish and saltwater fish. Those are very different from each other. For example:
"Saltwater fish tend to lose water because of osmosis. In saltwater fish, the kidneys concentrate wastes and return as much water as possible back to the body. The reverse happens in freshwater fish, they tend to gain water continuously. The kidneys of freshwater fish are specially adapted to pump out large amounts of dilute urine. "
Some – but only some – fish have specially adapted kidneys that change their function, allowing them to move from freshwater to saltwater.
There are a number of other differences.
The problem with a global flood is that the level of salt in the water (salinity level) is the same all over the globe.
Some marine species are very sensitive to salinity levels. Many aquatic plants are also very sensitive to the level of salt in the water.
If you think that this is not a problem, call your local aquarium dealer and say that you want to have both freshwater and salt water fish kept in the same tank. I’m sure that you can guess the reaction that you’ll get.
With this background, the question becomes a bit more difficult to answer than we first thought.
Do creationists have an answer to this question?
Here’s one answer from the well-known creationist web site Answers in Genesis:
"Well, most fish didn't survive. In fact, if you'd been a diver in the oceans before the flood, and then you'd been saved on the Ark and had started diving again after the flood, you would've said something like, "What happened? Where's everything gone?" You see, most marine species were killed during the flood. Now certainly some fish did survive, and we see their descendants in the oceans today. Some people then ask a related question; 'How did freshwater fish survive in the saltwater oceans?'There are two possibilities. First, there are many areas in the world today where we see freshwater and salt water together, and the two waters don't mix. So it's possible that certain organisms survived in pockets of fresh or salt water. Second, because of natural selection, which creationists accept, organisms today have become very specialized. Organisms at the time of the flood, however, would've been much stronger and able to tolerate many more changes than they can today. There's really no problem at all in answering this question.
Well, gee. Is it really that simple?
No, it’s not.
First of all, the flood waters were “raging”. The same Answers in Genesis web site referenced immediately above describes the flood of Noah like this:
"During the Flood, raging water, enough to cover the entire earth, transformed the landscape, laid down layer upon layer of sediment, and formed most of the fossils that we are still digging up today. The earth shows evidence, not of gradual change, but of one cataclysmic event that completely transformed the world and left us with an amazing testimony to the power of our Creator."
Though the waters were “raging” and causing “cataclysmic” damage, some parts of it were supposedly so still and gentle that the waters didn’t mix together and isolated “pools” were formed. That’s a clear and obvious inconsistency.
Besides, it should be an easy experiment to demonstrate that saltwater and freshwater fish can live together. It shouldn’t take any more than a swimming pool, some salt and a year’s time.
Of course simply showing that some fish can survive will not be enough. You would need to select those fish that are the most sensitive to salinity levels and show that they can survive for a year.
Furthermore note that when AIG brings up the idea of evolution through natural selection as the reason for the many types of marine species we see now! But the rate of evolution required for so many species in just a few thousand years has never been witnessed. In fact the rate of evolution is faster than a few thousand years! If it had been continuing at the required rate into the 21st century, we should see it demonstrated all around us.
We don’t. So lots of evolution took place in a short time. How short? Presumably in just a few centuries. But what caused it to stop?
Evolution explains a lot of things in nature. It doesn’t explain this.
Another problem for creationists relating to marine animals and the flood is the very narrow biodiversity of freshwater fish.
Of course all of the oceans are connected so saltwater fish can exist all over the Earth. We see this of course. Probably the most well-known example – sharks – are present in all of the oceans.
But the limited biodiversity of freshwater fish is much more difficult to explain.
Consider one well-known species of freshwater fish – the Piranha. Piranhas are found only in the Amazon River basin and no where else.
Yet, because they are a fish, during the entire year that there was a global flood, they could have swum wherever they wished to! The climate in Africa in many parts of Africa is identical to that of the South American jungles, so why didn’t at least a few of them swim there across the Atlantic Ocean? Surely given a full year to swim there, they could have done so. Or they could have swum to Florida or even Central America.
Yet they didn’t do so. Why not?
If Piranhas were the only species with this problem and the other freshwater fish were found abundantly everywhere that the environment suited them, we wouldn’t find the problem as perplexing. But of course the opposite is true. The vast majority of species of freshwater fish are found in very localized areas. Those found in multiple places were generally intentionally introduced by humans as game fish.
There appears to be no explanation – except that there was never a global flood.
 http://en.wikipedia.org/wiki/Fish, referenced on April 24, 2008
 http://www.answersingenesis.org/Home/Area/feedback/feedback26-feb-2001.asp, referenced on April 24, 2008
 http://www.answersingenesis.org/get-answers/features/noahs-ark, referenced on April 24, 2008