my belief:
so the reason for the flood, and why noah survived with his family, was that genetic impurities were romaing the earth. from the time of adam and eve's exile to the flood (1600 years iirc) shit got fucked up majorly. people started fucking animals, demons started fucking people....this is backed in archeology and anthropology and the fossil record. this is where dragons (dinosaurs, as retards call them lol) came from, hercules, minotaurs, spinxes, egyptian gods with coyote heads... all that shit.
so these entites were all over the globe, to the point where literally, noah and his family, were the ONLY pure humans left, gentically. so god had to kill everything. from what ive seen, god cares a LOT about genetic purity...thats why he was having david and saul wipe all these tribes and dudes out, for example, god tells David to destroy the amalkites and leave nothing living. Kill all residents and animals. Looking at secular research of the amalkites...they were sorcerers that specialized shape-shifting specifically into animals...which means animal sex as well. God had David annihilate everything to completely obliterate any unclean genetics. Saul was commanded to do something similar and kept some cattle alive for sacrifice and god took his empire from him for it.
but the flood only killed things on the land. these abominations that thrived in the oceans survived.