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Set in the early 21st century AD, the entire world is hit by a massive flash flood. The entire surface of Earth is covered by water with an average depth of 300 meters, and less than 10% of the human population survive the calamity. Persistent thunderstorms and severe weather conditions increase the death toll exponentially. Those who are fortunate enough to survive must source for freshwater and food else the extinction of human race is inevitable.

My question is where can we find freshwater in such a scenario and will humanity recover?

Note: This flash flood is the consequences of several magnitude 9 earthquakes simultaneously occurring throughout the globe. All mountains were flattened, and all valleys were filled with landslides. 100% of every surface is covered with water.

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    $\begingroup$ What about the mountains? Are they also below water? $\endgroup$ – clem steredenn Jul 31 '15 at 9:12
  • $\begingroup$ Waterworld, is that you? en.wikipedia.org/wiki/Waterworld $\endgroup$ – Theik Jul 31 '15 at 9:15
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    $\begingroup$ So you had an event that flattened all mountains. And you have human survivors... Well, i doubt that. But those who did should learn to boil their water. They also need to learn how to build a boat from nothing in absolutely forbidding conditions. And catch fish with the equipment they built from nothing in the same forbidding circumstances. I would say they are dead. All of them. $\endgroup$ – Burki Jul 31 '15 at 12:27
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    $\begingroup$ Wait... We have to deal with earthquakes, flooding, and exponentially-increasing trolls? I wish I could run for the hills, but there aren't any... $\endgroup$ – Frostfyre Jul 31 '15 at 12:50
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    $\begingroup$ Your scenario predicates that humanity = dead. $\endgroup$ – James Jul 31 '15 at 15:16
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Freshwater

Assuming the several magnitude 9 earthquakes, resulting tsunami's, and general destruction somehow didn't kill you and destroy any flotation device; there will only be 4.5 viable ways to get fresh water.

  1. Desalination- Involves heating saline water, collecting the steam/condensation

  2. Polar Ice caps- Glaciers store about 69% of the world's freshwater, so find a hunk of ice and melt it.

  3. Freshwater underwater- There are freshwater springs under the ocean, find one and stick a straw in it. The article/reference mentions:

    [there is] an estimated half a million cubic kilometres of low-salinity water ...buried beneath the seabed on continental shelves around the world.

  4. Rain- water that would only require a bit of work to make it drinkable.

.5 Any sealed water container filled with fresh water pre calamity and unbroken.

Will humanity recover?

No, unless there are additional magnitude 9 earthquakes that reform landmasses without killing off the original survivors (of which there need to be at least one male and one female that are viable partners).

This disaster would revert the world back to the stone age basically (further probably) since there would be no electricity, no TP, no gas, no tools (unless you saved a towel or hammer(which doesn't float very well)), no fire, no agriculture, etc.

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    $\begingroup$ Additionally, rain water is fresh water. It probably needs to be boiled, but it is or can trivially be made perfectly drinkable. $\endgroup$ – a CVn Jul 31 '15 at 13:48
  • $\begingroup$ @MichaelKjörling that's true, it would require large containers to collect enough drinkable water, though. $\endgroup$ – depperm Jul 31 '15 at 13:51
  • $\begingroup$ you would need far more then one male and one female to survive, inbreeding and all that. Also, as I said above, we would die of survey within a year, even if land masses came back a week after they went away there is not time to start growing new fruit before we died of lack of appropriate nutrients. $\endgroup$ – dsollen Aug 3 '15 at 14:15
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If humanity survived...

I'm going to assume the most basic technologies, since the thought of being able to extract groundwater or desalinate (mechanically) the ocean would be unlikely. This is just elaborating from depperm's answer, and adding some more suggestions.

Flotsam

Your folks on the few boats should have a little bit of water in the first days. My boat has a 220 gallons of freshwater on board, though I doubt she'd survive what you're describing, or that I would be able to get to it. Add to our boats any salvageable water containers (?) that have enough boyancy or air to float. There will be a LOT of flotsam: I'm swimming to the nearest semi-capsized or afloat mega-yacht.

Natural Desalination

A solar still, as pictured below, was available to air force pilots, if I remember correctly. At any rate, some tubing and plastic means that on a hot day, you can get pure water from evaporation.

enter image description here

Rainwater

This is tricky, but not impossible; on high seas, this might also imply rough weather, though. As mentioned in one of the other answers / comments, in a pinch it can be drinkable if you have vessals large enough to store it.

Dew

You can collect the morning dew easily with any (clean) bed linens, plastic or flax, by stretching it an angle from your boats or whatever you're clinging to. Much of the dew will drain downward into your container or you can squeeze the linens to get it before the sun does. The sides of our tents, even in the desert, would be soaked in the mornings with dew, so whatever fabric that was, go for it.

Alcohol

With salvaged water supplies, you can help (if you can't boil) by adding an alcoholic beverage to it. As an added bonus, if you don't go overboard, a moderate consumption of beer can be hydrating. I can confirm from ..um.. research.. that unopened beer cans float.

Now What?

You will have to answer a lot of problems, not the least of which includes food, humanity, and also: where did 300m+ water come from?!

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