A global flood would be quite impossible, there just simply isn't enough water.
There's enough water, you'd just have to change the topography of the Earth for everything to be covered.
To "briefly" summarize the
Story of Prophet Nuh written by Ibn Kathir (and based on the Qur'an and ahadith):
People were saddened by the deaths of a few pious people, so Satan inspired the people to erect statues to remember them. Generations later, the people had no idea what the statues were for, so Satan inspired them to worship these statues as gods, believing that they would bring them good, protect them from evil and provide them with all their needs. The idea basically spread from there. They forgot about God, and subsequently became ignorant, cruel and immoral.
So, God sent Noah as a messenger to his people, calling them to worship only God, and guide them to the truth. Most of his people rejected him. Aside from his family, he was only followed by the poor, weak, helpless, etc. among his people, while everyone else rejected him, and he was opposed by the rich and powerful elite, with taunts and threats. He argued with them, and kept calling his people for centuries. Every passing generation told the succeeding one not to believe Noah and to wage war against him.
When his followers were no longer increasing, he became disheartened with his people, but didn't despair. God then revealed to him that no others would believe and not to grieve for them. So Noah prayed to God to destroy the disbelievers, not leaving a single one on Earth, saying that they would mislead the believers and only beget more disbelievers. God accepted Noah's prayer, and told him to build a ship with his knowledge and instructions. He began construction on the ark, away from the sea. Those who rejected him, mocked him over it. When he finished the ark, God revealed to him that when water miraculously gushed forth from the oven at Noah's house, that would be the sign of the start of the flood.
The sign came, and he, his family (except for 1 of his sons) and followers embarked on the ark with 2 (male and female) of every kind, for which his people laughed at him for. Water rose from the cracks in the earth; there was not a crack from which water did not rise. Rain poured from the sky in quantities never seen before on earth. Water continued pouring from the sky and rising from the cracks; hour after hour the level rose. The interior of the earth moved in a strange way, and the ocean floors lifted suddenly, and the seas and waves invaded the land.
When the decree was fulfilled against the disbelievers, God commanded the earth and the sky to withhold its water. The water diminished, and the ark rested on Mt. Judi, at which point, everyone disembarked.
We see a suggested change of the topography, with the ocean floors lifting suddenly. With such a tectonic phenomenon, a lot of ocean water would be displaced onto the land.
The average depth of the ocean is approximately 2 miles, with the depth of the trenches being approximately 5 miles.
Currently, water already covers 70% of the Earth at the average depth of 2 miles. Hypothetically, if the ocean floor lifted up to sea level, then all that ocean water would be displaced above it, and you'd have 2-mile high walls of water, crashing down onto the shores. The water would disperse itself over the remaining 30% of the Earth, lowering the water's average height above it by 25%, making it 1.5 miles (approximately 8,000 ft.) over the rest of the Earth.
So, while there isn't enough water on Earth or in the atmosphere to flood the entire world at its current topography, if the topography were changed, there would be.
