If the water in the oceans receded, could we walk ...

There are approximately 1375 million cubic km of water on the planet. The amount of water present is enormous. So if we remove that much water, the Earth is a gigantic desert, and the journey from Europe to North America would be a very, very long journey.

actually no. You're unlikely to be able to cover that distance, even if you have a caravan of provisions. The mid-Atlantic ridge would be a challenge to be overcome. So unless you were an experienced climber and could avoid places where lava would turn you to ash, you probably wouldn't have made the trip a success.

What would happen if all the water in the oceans were replaced by deuterium oxide? - Ask me anything

Deuterium oxide, also called heavy water, is a chemically indistinguishable substance from water, it is therefore drinkable, it quenches thirst and its