I can't tell who's shitting us, and who's for real here... so I'll jump in with abandon anyway.

First and foremost, a huge percentage of the worlds ice is on land. An absolutely massive percentage of it. When that melts, it's going to follow the path it's liquid cousin already follows, and flow down into the sea (some might flow into lakes, though, but that's its problem... stubborn bastards). Well that can't be too bad, can it? I mean, we have water flowing into the world's seas daily! Yes it is a problem - people don't realise the sheer volume of ice. Take glaciers, for example. They can be miles long, and hundreds of meters thick. The same goes for ice sheets, except that they can cover thousands of square miles. Only two exist, in Anartica and Greenland, and if they melt, then we're buggered. If the Greenland ice sheet melted completely it would cause sea levels around the world to rise by 20 feet (and completely re-direct the Gulf Stream, leaving little old Britain kinda cold), and if the ice sheet in Antartica melted, sea levels around the world would rise by 210 feet. That's a lot. Now I'm lucky, because I live in a very hilly country, and I'm over 1,000ft above sea level, so it'll take something stupid like the whole country sinking to immerse me in sea water, but there are a lot of places worldwide that would be completely flooded with only a six foot rise - take Holland, for example. Extremely flat. An extra six feet of water? Bye bye you crazy Dutch.

So to summarise - a lot of ice is on land. When that melts, billions of gallons of water would flow on their merry way to the sea, and Holland would drown.