Is that even possible? Is there even that much ice on the planet?
Sure, it’s possible, since sea levels have been both much higher and much lower in the past. Still, we’re in an interglacial period, and it’s just as possible that the ice sheets could start growing again and the sea levels would drop, opening up coastal areas that have been underwater for millenia.
I think I’ve seen the calculations, which are probably close to correct, which suggest that if there all the worlds ice sheets melted the total sea level rise would be in the 200-300 ft range. Really, the sea level rise at the end of the ice age was on the same order of magnitude - so there’s nothing too unbelievable about that number.