What is the significanct difference between land-based ice and water-based ice (besides location)?

Question

from Al Gore's An Inconvenient Truth

Answer 1

Ice that is already on the ocean will not contribute to sea level rise as a consequence of global warming.  (Melting ice cubes in a drink do not raise the level of liquid in the glass.)

Ice that is currently on the land, however, will raise sea levels when it moves into the ocean, either as ice or as water.  More total water is in the ocean.