ice floats cause its specific weight is lower then water, water is the only substance that increases its volume when in solid state. The "over the surface" portion of ice represents exactly the extra volume increased by the solidifying event. Antarctic ice doesn t go over 1m depth, water covered surface of earth is 510,072,000 km², southern summer-ice pack surface is around 5,000,000, that makes 1cm of sea level increase in a really disastrous event.