Waterworld/WMG

Waterworld would've been the future of Pokémon Sapphire, had Team Aqua suceeded.
Their plan was to use Kyogre to flood the Earth.
 * Conversely, The Postman would've the future of Pokémon Ruby, had Team Magma has suceeded in their plan to use Grodoun to drought the the world.

A massive ice asteroid caused the rise in temperature and the sea levels.
The main Just Bugs Me about the film is that there simply isn't enough water on the planet, solution? Just add water. A large enough ice asteroid would explain why the sea level is so high, it also caused the mass extinction of most of the other types of creatures on the planet which is why we don't see many. Also maybe the asteroid contains lots of sodium to explain the salt levels in the water.
 * Ice asteroids are way too small to seriously increase the amount of water to such levels, and would be far more destructive. If the world's water levels increased by 300 meters, and we know it was more than that, then the asteroid would have to be 330 kilometers in diameter. 36600 times more destructive than the one that wiped out the dinosaurs. It would break the crust, it would cause a nuclear winter, it would produce earthquakes greater than have ever been seen in the last four billennia. It would wipe everything and everyone. Nobody could survive it.
 * Not to mention, an asteroid would probably throw up enough debris into the atmosphere to cause a new ice age, which would cause the ice caps to expand, and make the water level around the world go down.
 * According to My Calculations, in order to actually get the sea to rise 25,000 feet (which would cover all the continents except for a few mountains in the Himalayas which, logically, becomes Dryland) it would take a single comet over 2,000 kilometers in diameter. Impacts with stuff like that gets into Earthshattering Kaboom levels.
 * Well there goes that theory. How about a series of smaller asteroids over a longer time-span. Highly unlikely granted but this is WMG.

Aliens dropped off the water.

 * Maybe intentionally, maybe unintentionally by an AI terraforming craft, but instead of the extra water being deposited by comets (which are destructive), the terraforming ship picked up a few comets, melted them, added the needed salinity and pumped the water down to the surface via a temporary Space Elevator. Aliens will get here when they get here, point is Earth is stuck with more water than we can shake a stick at. (Hey, if the Battle: Los Angeles aliens can reduce the sea level in a day, then these guys can pump it up.)
 * Thorny is why we didn't fight it off. I'm guessing that the water was pumped down in such quantities and so fast coastal communities didn't have a chance, and think of how many National Capitals are near sea level, too.

This is in the same world as Mad Max
Australia just happened to be unharmed.

The ocean isn't global
There's whole continents still around, just nowhere near where these folks are.


 * THAT... explains quite a bit.

Waterworld takes place in the in the Fallout verse.

 * think about it we never actually see the oceans in fallout and the Pirates in the movie were vary Raider like. mayhep the world was not flooded completely perhaps this is happening in some god forsaken stretch of oacen between the east and west.