Question about melting polar ice caps?

Your experiment duplicated polar sea ice, not a polar ice cap. Take a pie plate, put 1/4 inch of water in it, set a cup upside down in the middle of it, and put an ice cube on top of the cup. As the ice cube melts, the water level will rise. The ice caps on Greenland and Antarctica, if they melted, would increase the world's ocean levels hundreds of feet if they melted entirely because they are sitting on land, not floating.