Globally
Global Mean Sea Level increased by 195mm between 1870 and 2004, and is continuing to rise at a fairly steady rate of just over 3mm/year. On average, it is expected that by 2100 sea levels will have risen in most places by around half a meter. In many places, 50 centimeters would see entire beaches being washed away, together with a significant portion of the coastline.
Locally
For people living on low-lying islands such as Tuvalu, Kiribati or the Maldives, where the highest point is only 2-3 meters above current sea levels, an extra 50 centimeters could see significant portions of their islands being washed away by erosion or covered by water. Even if they remain above the sea, many island nations will have their supplies of drinking water reduced because sea water will invade their freshwater aquifers. While these islands have sizeable populations, they’re insignificant compared to the tens of millions of people living in the low-level coastal areas of southern Asia. These include the coastlines of Pakistan, India, Sri Lanka and Burma. Densely populated areas like the Nile Delta and parts of Bangladesh would become uninhabitable, potentially driving hundreds of millions of people from their land. The sea level rise is exacerbated in some areas where large-scale land subsidence causes a rate of rise relative to the land which is substantially higher (e.g. Gippsland, Victoria and the Gulf coast of the U.S.). Correspondingly, some areas have seen less impact because they are rising. Australia is rising at about 0.3 -0.4mm/year due to Glacial Isostatic Adjustment (GIA), but is still starting to feel the effects.
A sea level rise of only a few meters would inundate thousands of square kilometers of highly populated and developed land. In Amsterdam, London, Rotterdam, Portsmouth or other low-lying coastal areas; communities, highways, factories, ports, railroads and many types of industry would be flooded.
The complete melting of Greenland would raise sea levels by 7 meters (23 feet). But even a partial melting would cause a one-meter (three-foot) rise. Such a rise would have a devastating impact on low-lying island countries which would be entirely submerged.