I adore debunking the flood story!
Every ancient culture has some story of a flood probably because floods can happen and do happen everywhere. These days we know why this is but in the stone age or bronze age they made up stories to explain it.
For the sake ofargument though, if there really had been a global flood, which moved the continents/mountains/valleys to their current position ( as JW's claim) that would have destroyed every last land growing vegetation. The ground afterwards would be a wasteland of mud, sand and rocks. No seeds or bulbs would have survived and it is doubtful that the ground would be in a fit state to grow anything in.
In short there is no way that a olive tree could have survived for Noah's dove to pluck a sprig off!