Suppose that instead of a major river there was only a bay between Manhattan Island and New Jersey extending inland as far as the junction?
It would not be exactly the same. For example, since commuting to Rockland County would be easier, more Manhattan workers would live there, and fewer would live in the outer boroughs. Oh wait, it would be totally different. Without the river, there would have been no way for boats to get between the Erie Canal and the Atlantic Ocean. So New York State would still be a relatively rural area (similar to Delaware or Rhode Island), and Philadelphia would still be the major city between Boston and Maryland, like things were before the canal was built. Rockland County would be about the same as it is now -- except without so many NYPD officers living there, because there wouldn't be so many NYPD officers, and Westchester County and the outer boroughs and Long Island would also be like Rockland County now, again except with fewer NYPD officers.