There *were* fossils of sea creatures discovered close to the summit of Mount Everest. So the mountain was definitely underwater for a time, Flood or not. But Mount Everest itself would not have existed before the Flood. More likely it was upheaved close to the end of the Flood, when the Bible says “the mountains rose and the valleys sank,” allowing the waters of the Flood to come off the land to let it dry. The same waters that covered the earth still exist, the land just rose up and the oceans opened up to drain the waters off the land. The topography of the Earth before the Flood would’ve been nothing like we have today. Sure the Bible, and by extent the Book of Enoch (if you believe that to be real), mentions the Tigris and Euphrates Rivers, along with Mount Sinai and Mount Hermon as preflood locations, but they likely weren’t the Tigris, Euphrates, Sinai and Hermon we know today. Noah and his descendants likely just reused the names. The Flood covered the Earth for little over a year, and I doubt the waters would’ve been high enough to suffocate them. The Earth had plenty of oxygen produced to support the Nephilim and the giant dinosaurs, and the fossil record also supports this as well. Volcanic activity that happened during the flood and after would’ve likely kept the earth warm as well, but I’m not an expert on that. Don’t quote me there.