There were at least two Cornelius Seaburys born in Tiverton around the same time, and they were first cousins. One was born 7 April 1763, a son of Aaron & Abigail (Westgate) Seabury. The other Cornelius was born 2 May 1769, a son of Philip & Sarah Seabury. Based on his estimated date of birth as given in the RI Cemetery Database (1769) and also based on his being named in his grandfather's will (appearing to be among the five sons of Philip, listed together), I have made an assumption that he is a son of Philip & Sarah.
These five sons of Philip & Sarah are also mentioned as grandsons in the will of Marcy Pearce, widow, of Tiverton; here they are identified as sons of Philip Seabury.
This conclusion is confirmed by the marriage record:
SEABURY Cornelius, of Philip and Sarah, and Mary Gray, of Col. Pardon and Mary; m. by Rev. John Briggs, March 16, 1714.
From the RI Cemetery Database:
SEABURY CORNELIUS 1769c - 1 SEP 1854 TV011
SEABURY MARY (GRAY) 1771c - 27 AUG 1847 TV011
There is no other Cornelius listed in the database for the earlier date of birth, which suggests that he may have died young or moved out-of-state.