What's way, way worse than sea levels rising is the fact that the oceans would get warmer and more acidic. Warmer and more acidic is bad because it kills sea life (especially sea life that use carbonates for skeletons). Warmer water also has less of an affinity for carbon dioxide, meaning that you lose a carbon sink both from higher-temperature water and also from the death of sea creatures that store away carbon dioxide as calcium carbonate.
If this went totally out of control, we would essentially lose one of the biggest global carbon sinks, and we would be pretty much forgeted because carbon dioxide levels would raise very quickly.
