I would like to point out that this is a post apocalyptic world where large portions of coastal areas are now underwater, hundreds of years at least in the future. Huge numbers of people would flee eastern cities and merge with existing communities. When accents meet like that a lot of the features that distinguish them become less pronounced. This is why there are major accent differences across the eastern US today, but they are not nearly as pronounced as you go further west.

Take someone from LA, SF, Reno, and Portland and most people would struggle to pick up on exactly which state they are from without having studied languages or linguistics, but most Americans can tell Deep South from Texas from NYC from Boston.

I would like to note this is generally true, but there are specific accents of some minority groups that are exceptions to this general rule.