According to the U.S. Geological Survey, there are over 332,519,000 cubic miles of water on the planet. Remember this is cubic verses 225 square miles. So we're talking about a drop in the ocean.
so approximately a 7.5 to 8 cubic mile piece of ice(assuming a height to depth 200 feet average with .9 of the ice under water) broke off...since that ice was already floating on the water there would be no positive displacement.