“Ok do this. take a glass of water and put a large rock at the bottom. Then fill with ice. Let the ice melt. What do you see? It is the exact same displacement.”
Your analogy is flawed. Fill a glass of water. Place a screen on top of the glass. Place a large ice cube on top of the screen so that when it melts the water drips into the glass. What would you see?
Not proper analogy unless the ice is elevated 100 feet in the air over the arctic sea.
His analogy is only partially flawed. His analogy is correct for sea ice... melting sea ice would not substantially affect the world’s sea levels one bit, since its already displacing its water weight in volume as it floats.
Your analogy is correct for land-ice, mostly. Ice on land melting into the oceans would raise the sea levels, though there would be an additional, unpredictable effect based on the fact that the land would rise a small amount without the weight of the ice upon it (such as happened with the North American continent after the glacial retreat following the last ice age).
That said, even if ALL the ice on earth melted (and even the most extreme ‘Global Warming’ scenarios would not raise temperatures enough to create a totally ice-free globe) you wouldn’t get ‘Waterworld’ ... there’s just not enough water. Sea levels would rise about 200 feet. And more realistically, since Antarctica (where 90% of the land ice is) stays pretty much well below freezing always, even under an extreme Global Warming scenario, it’s still going to be frozen. Greenland is a different matter, but that’s a lot less ice involved there ... it might raise sea levels by about 20 feet. That would kind of suck for coastal areas, but remember that it won’t happen in one giant tsunami... ice takes time to melt, especially if the temperature is only barely above freezing... the time it would take to ‘melt’ Greenland would likely be longer than a thousand years.
In short, the crazy alarmist-type movies you see of New York vanishing under a giant wave are not science, they’re fiction.