While it is possible that the ABC poll mentioned may have had flaws, ABC is an experienced polling organization. It is unlikely that it has made an error of tens of percents in the estimates. The given percentage is 61, with a margin of error of about 3%.
This is in line with other polls. This one gives 54% as the number who believe in a six-day creation. This one gives 55%. This one says 40%. This one says 66%.
So while the precise figure is hard to determine, and probably depends on the question, is is clearly true that something in the region of half of Americans believe in the literal truth of Genesis. And by implication Noah's Ark.