Richard Dawkins explains the IMMENSE problems creationists have in explaining the diversity of land animals. But, possibly surprisingly, that problem also exists for aquatic animals - most specifically those that live in freshwater.
One problem for the flood, of course, is that no creationist that I've ever seen believes that Noah took any aquatic life forms onto the Ark. But, in fact, aquatic animals live in EITHER freshwater or salt water but, with very few exceptions, not BOTH. In fact many animals are very sensitive to the salinity level of the water. If you have a home aquarium in which you are planning on keeping salt water aquatic animals, you are given very specific instructions on what salinity level to keep the water at. Coral reefs are more sensitive than most other aquatic life forms.
For example, the web page at http://www.saltwater-aquarium-online-guide.com/acclimation.html says:
"Maintain a salinity level of 1.019 – 1.022 for fish only aquariums and 1.025 – 1.028 if you have corals and/or clams."
Note how specific and narrow those ranges are.
If you doubt any of that, call you local aquarium and say that you want to keep freshwater and salt water fish in the same tank along with a coral reef. Warn them ahead of time, however. They will probably literally fall on the floor laughing.
So **IF** the flood occurred as described in the Bible we should have seen pretty much all aquatic life would have died. At a minimum all coral reefs would have been destroyed. (Maybe a few salmon might have survived.)
We don't see that.
But, I'm here to talk about biodiversity.
Because all of the oceans are connected to each other we see salt water organisms all over. You can find a shark in any ocean.
But that is NOT true for freshwater fish. The reason is obvious - freshwater fish necessarily live in isolated waters. Even rivers don't run from Africa to Asia.
But **IF** there was a global flood and in the VERY unlikely event that it didn't kill all freshwater animals outright, then for a year they would have had the same ability to diversify that all saltwater fish have.
Evidently they didn't. There are many, many examples of freshwater organisms that stay in very narrow ecological niches.
Arguably the most well-known freshwater fish is the piranha. It lives in a very narrow ecological niche - the waters of the Amazon river basin.
Why would that be so if there was a global flood?
According to the Bible, the flood lasted a bit more than a year.
In a year piranhas would have been able to easily swim to Central America and places like Florida. Given a year, a trip to Africa would have been a reasonably leisurely swim. All of those places have the sort of climate that piranhas find ideal.
In fact, such swims should have been even more leisurely. That's because many, probably most, creationists claim that the plate tectonics that we see evidence for took place during the flood year. That means at the start of the flood Africa was actually connected to South America. It also means that piranhas and other fish wouldn't even have had to swim much at all. They should have been really carried along with the land itself.
But despite all of that, piranhas stayed in a relatively small geographical area.
Surely no answer will be found in the Bible.
Of course piranhas are far from the exception. They actually represent the rule. A very large percentage of freshwater aquatic organisms live in similar small niches.
One final related problem for creationists - why are there NO fossils of freshwater fish mixed in with salt water fossils? **IF** they lived together and **IF** the flood created the fossil record, what possible explanation is there for the complete lack of such fossils?
The unanswerable questions for the flood account in the Bible go on and on and on. That's no doubt why the ACTIONS of most creationists - which speak louder than their words - show that the DON'T actually believe in the flood of Noah.