The ice cube's submerged portion occupies the volume of water that is equal to the weight of the ice. Density is irrelevant. If the ice displaces 2 pounds of water, then the ice weighs 2 pounds, and therefore will not displace less or more than that when it melts. Try this: add some ice to a glass of water and mark the meniscus. Check it after the ice melts and if it's any different, let physicists know because you've just done some incredible voodoo witchcraft.