Dear Hannah,
Thank you for your question. The simple answer is no. The whole world will never be underwater. But our coastlines would be very different.
If all the ice covering Antarctica , Greenland, and in mountain glaciers around the world were to melt, sea level would rise about 70 meters (230 feet). The ocean would cover all the coastal cities. And land area would shrink significantly. But many cities, such as Denver, would survive.
However, all the ice is not going to melt. The Antarctic ice cap, where most of the ice exists, has survived much warmer times.