John Gill Commentary Numbers 34:6

"And for the western border, ye shall have the great sea and the border [thereof]: this shall be your west border." — Numbers 34:6 (ASV)

And as for the western border
Of the land of Canaan:

you shall even have the great sea for a border ;
and no other, meaning the Mediterranean sea, which lies west of the land of Judea; Aben Ezra calls it the Spanish sea: it has the name of "great", in comparison with some in the land of Canaan, as the salt sea, and the sea of Tiberias:

this shall be your west border ;
namely, the Mediterranean sea.