The WTB&TS repeatedly show thier ignorance on this subject. There are nearly 60,000 species of vertibrate animals described. There are over 1.2 million species of invertibrate animals, nearly 300,000 species of plants, and nearly 30,000 species classified as "other" (fungi, lichens, brown algaes, etc.). So the total number of described species is nearly 1.6 million, and we all know there are more yet to be discovered. [Edit: These are old numbers, no doubt higher now]
There is a chance that some of the plants may survive immersion in water for that long, not many though. And there would not be a seperation of saltwater/fresh water (there may be some gradation, but it would mix) and very few fishes have a tollerance range that would allow them to survive.
If you look simply at the number of species, you know the myth of the flood is false. The arc wasn't even very big, although well beyond the capability of its purported builders.
I have argued this with several JWs, and they claim that the species "diversified" after the flood, which for one thing calls on evolution. That could easily be disproved looking at DNA/RNA.
I can show you 15,000 year old deposits from a small stream, yet a global flood didn't leave any evidence? It still shocks me that anyone can believe this.