The west border was the coastline of the Great Sea. This is the boundary of the children of Judah all around according to their families.
Joshua 15:12
Surrounding context
15:11 And the border went out to the side of Ekron northward. Then the border went around to Shicron, passed along to Mount Baalah, and extended to Jabneel; and the border ended at the sea.
15:12 The west border was the coastline of the Great Sea. This is the boundary of the children of Judah all around according to their families.
15:13 Now to Caleb the son of Jephunneh he gave a share among the children of Judah, according to the commandment of the LORD to Joshua, namely, Kirjath Arba, which is Hebron (Arba was the father of Anak).