I did a little more internet searching

You are right that the top of the Great Pyramid is roughly 200 meters above sea level

However, if all the world's glaciers & ice melted, internet search tells me it's only about 70 meters of sea level rise. The Giza Plateau is 30 meters above sea level, plus the Great Pyramid is roughly 140 meters tall, so the "wave erosion" should be at most 1/3 up the pyramid, not 2/3+