The fact that Polly Watson married Cornelius McEveny on 19 Feb 1792 in Williamstown, Berkshire, Massachusetts suggests she may have been born around that area. But it is equally possible that she was an immigrant. However, there were other Watsons that married there around the same time:
Abijah Bur married Sarah Watson on 18 Nov 1791
Ebenezer Watson married Cynthea Wilkinton on 20 Mar 1792
Joseph Crofoot and Lois Watson on 27 Aug 1801
There is also a Daniel Watson that married (wife's name unknown) and had children:
David, b. 13 Dec 1778
Noah, b. 30 Dec 1780
Anna, b. 26 Feb 1783
There is no way to know how these individuals may be related.
But with Polly being born in 1774 and Julia being born in 1848 there could be from one to three generations that separate them, IF they are related.