You can't just submerge plants in water for extended amounts of time and expect that they're going to be okay. Yes, plants like water, but as any experienced gardener will tell you, it is possible to kill them by overwatering. Quite possible.
Floods - even normal, regular, non-global floods - can be devastating to plant life.
You also have to take into account that light can only penetrate so far underwater - most light can't get more than about 200 meters through water, and that's assuming that it's clear, which it probably wouldn't be in the flood. So most of the plants on the planet wouldn't be getting any sunlight for the better part of the year.
So, we take into the account extreme flooding conditions, the lack of sunlight...oh, and lest we forget, the extreme crushing pressure from being covered by miles of water. I would say...yes, Noah should have walked out to a barren wasteland.
You don't find any plants at the bottom of the ocean. There's a reason for that.
You don't even really have to do that. If you have a lawn, you probably have one or two spots on it that pool up with whatever whenever it rains - I know my lawn does. There's a particular spot that always gets flooded after any significant rain. Nothing really grows in it. Occasionally a few blades of grass might start to pop up, but then it just gets flood again.