I agree with Firebrand (^above) that if the ice is already floating (sea ice) and it melts it will cause no other immediate displacement of volume (even if a large part of that floating ice is above sea level and even if solid water takes up less space than liquid water). That's an application of Archimedes principle and can be explained here: http://physics.stackexchange.com/questions/110645/why-does-i.... Indeed it's land ice melting that causes sea level rise. However, sea ice melting still changes the albedo and does directly contribute to climate change.

True, I was thinking that its not truly free floating sea ice and that much of it may be supported by the remaining ice its attached to (hence the crack).

Water in the form of a solid is less dense than as a liquid

Source: Put some ice cubes in water. Observe that they float.

Of course it floats. But that's not what I was trying to say. Go to your cup, put some ice in it and then mark the water level. Now let the ice melt and remark the water level. They will be at the same level.

What happens when you add an ice-cube? That's what happens when ice slides off of continents.

A break-up this huge may not directly alter sea-levels but it's certainly symptomatic of a larger disruption that's playing out on a massive scale.

Water takes up more space as a solid.