[Pontiac]

Sea levels will not rise one millimeter. In fact, the ocean levels would go down a tiny fraction because water contracts when it goes from solid state to liquid state.

You are correct if the water is floating like an ice berg. But this water is a glacier which is on land presently. If the glacier slides in to the ocean it will cause the ocean water level to rise.

Consequently the land on which the glacier resides will also rise (rebound) because of the weight of the water has been lifted off the land. Land elsewhere is likely to fall because this land has risen.

[mdmathis6]

I was given to understand that the ice was floating on water but still attached to the sheet until it got displaced from the ice behind it on land.