Genesis 7:17

And the flood was forty days upon the earth; and the waters increased, and bare up the ark, and it was lift up above the earth.

1. It took a while before the water rose high enough to lift up the ark - at least 40 days, perhaps more.
2. So, Noah and his family and all the animals sat in the ark in their backyard, so to speak, for perhaps a month and a half before beginning to float away.
3. Waiting for a month and a half must have given Noah and his family plenty of opportunity to question whether the flood really would be severe enough to drown everything and require the use of an ark to be saved.