Americans don't need to travel so far to see an ocean being born. The block-fault zone from the Sea of Cortez up through most of Nevada is pulling apart rapidly and has already created 500 miles of inlet between Baja California and mainland Mexico.

Much of the next few hundred miles north through the Salton Sea and Death Valley is already under sea level and all of Nevada has thinning crust that will eventually fall under sea level.

in fact, most of the valleys in Nevada, in spite of typical elevations around 4000 feet, are already below sea level. It's just that they presently don't have any outlet to the sea in the Great Basin so alluvium has collected as the fault-block mountains created by spreading erode. [0] When a river has a chance to cut down through that soft debris, the valley floors will quickly drop below sea level also.

[0] https://en.wikipedia.org/wiki/Fault_block#Fault-block_mounta...