Fla. Stat. § 7.45
The boundary lines of Nassau County are as follows: Beginning at the mouth of the Nassau River; thence northwesterly up the thread of the main stream of said river to the run of Thomas Swamp; thence southwesterly up the run of said swamp to where it would intersect the prolongation of a line drawn from the southwest corner of township one north, of range twenty-five east, to the southwest corner of township two south, of range twenty-three east; thence on said last mentioned line in a southwesterly direction to where its extension would intersect the range line dividing ranges twenty-two and twenty-three east and the eastern boundary of Baker County, all concurrent with the north boundary of Duval County; thence north on said range line and said eastern boundary of Baker County to the St. Marys River and the boundary line between the States of Georgia and Florida; thence north and easterly along the said river, concurrent with the said boundary line of the States of Georgia and Florida to the Atlantic Ocean; thence southerly, including the waters of said ocean within the jurisdiction of the State of Florida, to the place of beginning.
Fla. Stat. § 7.45