According to scripture, waters covered the earth for about 6 months. I'm not sure how many here have seen floods, but I saw rivers flood their banks many times growing up, covering all vegetation. When the waters abated, usually after a month or two at most, all plants and trees the water had covered was dead. The floods were usually in the spring and by the end of summer, greenery, several months later, had once again started to cover the flooded areas.
Recognizing that fact, how did an olive tree live covered by water for about 6 months?
Gen 8:9 But the dove found no rest for the sole of her foot, and she returned unto him into the ark, for the waters were on the face of the whole earth: then he put forth his hand, and took her, and pulled her in unto him into the ark.
Gen 8:10 And he stayed yet other seven days; and again he sent forth the dove out of the ark;
Gen 8:11 And the dove came in to him in the evening; and, lo, in her mouth was an olive leaf pluckt off: so Noah knew that the waters were abated from off the earth.
It was because the flood was a local flood.