It's not a "Waterworld" we need to be concerned about.
The worse case, 100 meter sea level rise, if we were to lose all the pack ice and glaciers looks like
THIS where Tennesee may see some bay-side property to it's west.
We don't need that dramatic a rise in sea level to cause major problems though. There are already small island communities in Micronesia, Bangladesh, Alaska, and elsewhere, where the small rise in sea levels seen so far are inundating their homes and property. A few centimeters can start causing some folks problems.
Or, you could plug in which numbers you want at
http://flood.firetree.net/?ll=43.3251,-101.6015&z=13&m=7 to see what effects different sea level rises have around the world. 6m (20 feet) is a common one.
Here are various maps for various rise in sea level.
EPA no longer officially updates it's global warming site, but they maintain maps about areas vulnerable to rise in sea levels
HereAnd there is a good map showing how low sea levels were 18,000 years ago during the last ice age, when they were 100 meters lower, compared to today, and compared to where they would be if all the glaciers melted located
here.
You have to remember that over half of the world's 6 billion people live in coastal areas, primarily large cities, lower ends of major rivers, and coastal plains. Any significant rise in sea levels that overwhelm any flood control projects that are built to protect these areas, and you get Katrina for up to half the world's population. A meter is enough to start things. Thermal expansion of water is not included in most models, but that can add to the volume of water (warmer water expanding, which raises sea levels even a bit more).
You also end up inundating most of the world's seaports and oil terminals. This greatly limits the ability to move bulk quantities of energy producing oil, natural gas, coal, bulk ores, grains and foodstuffs, and manufactured products.
You also end up inundating the desalination plants and lower river fresh water intakes and treatment facilities in much of the world.
You flood many major airports.
You flood many major routes for transportation and rail.
You flood a number of power plants.
There there is the change in evaporation and rainfall trends (Brazil sees this now, the Amazon River has dried up due to regional drought these past 3 years). There larger seas retain more solar energy in the form of heat. This can lead to stronger storms, or more storms. More damage done from these storms against crops and homes.
Finally there are the min ice age scenarios: less salty water in Great Oceanic Conveyor including the Gulf Stream leads to less heat brought to the North Atlantic = cooler temperatures regionally across New England, Eastern Canada, and Northern Europe. These areas experience much colder weather while the rest of the world heats up.
The 100 meter rise in sea level map above would be the result of a runaway event. The seabed floors and the permafrost in Siberia begin to thaw and release huge amounts of CO2 and methane, both greenhouse gases. That's the true runaway trigger, considering the amount of CO2 and methane sequestered in frozen seabeds and permafrost.
