At the beginning of Waterworld, we learn that the entire world is completely submerged
"The future: The polar icecaps have melted, covering the Earth with water"
However, according to this rather fancy video from BusinessInsider, if all of the icecaps melted, then sea levels could rise by as much as 15-20 feet, catastrophically placing many coastal cities and provinces at risk of submersion but hardly the entire world.
Where did the extra water come from to make the Earth into Waterworld? Was this discrepancy ever addressed by the makers of the film?