Your questions are very thought provoking in a investigative manner, the fishes in the Sea and rivers, should not have really had any problems, since water is their venue for living, it is when they are on the land, that they have some problems, for they were created to live in the water, and if Old Father Noah was told to get a pair of each creature in that Ark, then he would would have a very hard time getting those Sharkes into that Ark, since he would probably have to go out into deep waters, to get those sharkes as well as those whales, and what would he then do, catch then in a net, or speak a langauge which they understand and tell them to get into that boat, where there is no water for them to survive. And if the fishes in the Sea were all poisoned as you suggested, then it would mean, that these fishes which are existing today, would be new brand of fishes, which were created after the flood.

Now let us left the Sea and go back to the land, the boat had to be built before the flood, so it is there on dry land, and Noah had to find ways to communicate with all those Dog flees, Sand flies, Mosquitoes, Giraafes, Elephants, Lions, Tiger, Cats, Jaguars and so many more, who are mostly in the deep woods, that is a lot of work for a old man, let me hope, that he did not have to lift the Elephants, the Lions, the Giraafes, the Alligators and all those big and heavy ones, by himself, to put into this Ark, let me hope that they did jump in. I am also wondering, how would the Lions, Tiger Cats, Alligator and others who have to kill to eat, get along without trying to fight over killing the goats, Rabbits, Cows Zabras and other animals for their dinner, I imagined that father Noah must have had a very hard time, controling such a vast amount of creatures, who normally, did not get along, and then being housed in the same place, tightly packed together. I imagined father Noah had to watch his own neck and back, since lions, tigers, and Aligators, do not mind eating human beings. This is just a imaginative enquiry, as it relates to that same story, about Noah and his Ark, full with pairs of each creature upon the face of this earth, as he save them from the flood, which took place upon the whole earth. Feel free to add what you can, I am just searching for the truth.
Again I send many oceans of blessings and self determination to African people everywhere. ONE BLACK HEART ONE BLACK LOVE
Baba Ras Marcus