It doesn't work like that. The mass of water displaced is equal to the mass of the floating ice itself. Melting a piece of ice has no change on water level.
Both the southern polar cap and the Greenland ice mass rest on ground rather than float in the ocean, which means any melting will cause the ocean levels to rise.
I was talking about the displacement only (which the parent thread discusses). I of course acknowledge the common sense that not all ice is floating on water.