Stephen Oppenheimer has supporting data in his book Eden In The East. In fact, he says the coastline was even farther north and Ur was just on one side of the Gulf.
The idea that there was a gulf, or rather that the existing Persian Gulf used to extend well inland from its current location, used to be the consensus, and based quite logically on the estimates of silt quantities brought down by the rivers. But it has been proved that it isn't true.