How did 2 dog species since the time of christ turn into the many many many species we have today? Because of they were genetically rich. There were really only two types of dog only 2,000 years ago. Did anyone know that Noah was not in the Ark for 40 days. that is only how long it took to rain and flood. He was in there for 1 year. Vegetation survived 1 year under water not 40 days.
As for vegetation...many terrestrial seeds can survive long periods of soaking in various concentrations of salt water. Sure, salt water impedes the germination of some species so that the seed lasts better in salt water than fresh water.
Other plants could have survived in floating vegetation masses or on pumice from the volcanic activity. Pieces of many plants are still capable of asexual sprouting.
Many seeds attach themselves to animals, others could survive in the bloated dead carcasses of dead herbivores.
For Noah to know the earth was dry. a dove brought back an olive leaf.