How did Noah preserve fresh water fish? How did Noah prevent fresh water fish from dying when salt water inundated inland waterways during the claimed floods?  I'm not sure which Christian doctrines take the literalist position but what is their explanation for this aspect of the claim?
I realize that what I'm about to put forward is not conclusively proven, but it is possible.  It's specifically answering from the perspective of young-earth creationists, which isn't the only view out there. (But it is the one you specified you wanted in your question.)



Noah didn't have to save the fish.  

Your question makes two assumptions that seem reasonable, but aren't really axiomatic.


That the oceans were salt water at the time of the flood, 
That the fish at the time were already adapted to either fresh water or salt water.


It is entirely possible that at that early stage in earth's history (assuming a young-earth time-line) that the oceans were fresh water, or nearly fresh water.  If this were so, there would have been no dichotomy between fresh and saltwater fish.

The oceans gain their salinity because material from land is constantly eroding into the oceans.  Both sea water and fresh water evaporate, but all water eventually ends up in the oceans.  the reason the oceans are more salty than fresh water is that the minerals can't travel back upstream, so when the water evaporates, the salinity increases. (more here if you want it)

Even secular scientists are saying that most salt-water fish evolved from fresh water fish.  Even the young-earth creationists wouldn't argue that.  We see it as a minor variation within the Biblical "kinds"  Turning from a freshwater fish to a saltwater fish is a minor change compared to the transformation from a dinosaur (primarily reptile from what we understand) to a bird, for example.

One popular YEC view is that likely, at the time of the flood, most sea life was able to live in both ocean water and fresh water because the salinity of the ocean was much less than.  During the flood, large amounts of materials would have been deposited rapidly, and as the flood waters receded, the ocean would have been much more salty shortly after the flood than it was before from that fact alone.  Over the remaining years, the oceans have become more salty at a slower rate. 

A similar argument can be found here.

And another here, with a story about someone who experimented an was able to get salt and fresh-water fish living together in the same tank.  There's no reference on the story, but it's certainly possible to try it yourself if you'd like to experiment.

This article Fish in the Flood  has this to add:


  As a matter of fact, most of them didn’t survive. Over 95 percent of
  all fossils are marine creatures. They died and are fossilized by the
  trillions. Many are buried in great fossil graveyards, tightly packed
  together, choked with sediments, buried before they had time to decay.
  Obviously, they didn’t live in the environment in which they died.
  They were transported by rapidly moving water and then buried in
  sedimentary deposits.