Fish are killed by floods as I’ve observed. Muddy water and high amounts of sediments often choke fishs’ gills and fish are often trapped in pools of receding flood waters. Flood pools were always a good place to catch fish.
So you’re comment, “There should be no fossils of aquatic animals to speak of, since they wouldn’t have been killed, and therefore wouldn’t have left any fossils behind.”, is at odds with reality.
I've gotten conflicting information about what happened during the flood. Some say Noah didn't take any fish aboard the Ark, because he didn't need to. Others say the conditions in the oceans were such that there is very little if any chance that anything could have survive in them.