I'm starting this because "By Accident?" had been sidetracked.
For starts:
Yes the Flood occurred.
It was not local. It was world-wide.
It is remembered and mentioned in the stories of every ancient culture.
Only one family survived on a large covered barge-like boat.
With them were a number of animals who had the breath of life, or nephesh, (which is also translated 'soul') of the air and land.
All the animals and people on the Ark were vegetarians and food had been stored for them.
Insects, plants, amphibians all survived on what were probably gigantic floating vegetation mats, akin to what we see after the monsoon seasons in the Far East, only much larger.
The salt in the ocean was probably much less before the Flood, being more of a brackish salinity. In Genesis 7:11 we read that all the fountains of the deep exploded at the same time. This means the subterranean water was under enormous pressure and probably scalding. A number of ancient 'legends' talk about the explosions of scalding waters. This water, exploding upwards, would have carried with it enormous amounts of pulverized rock which, when saturated in the rainfall and subsequent flood, would have release great amounts of salt. However, this would not have been distributed even throughout the ocean, and still, actually, is not, as ocean currents have a great deal to do with the matter. Thus the original fish, given the genetic ability to vary which would have been in all original populations, would have been able to have some offspring survive in marginal waters and there truly would have been not only survival of the fittest at that point, but a great deal of speciation/variation.
It should be noted that even today we can be quite amazed at the ability of a number of kinds of fish to breed across what we consider species and even genus lines.
OK, that's enough for people to start shooting, I'm sure!