Why does the Torah insert the word "u'Gevul"?
Rashi (citing Gitin, 8a): To include the islands in the Mediterranean Sea 1 in the borders of Eretz Yisrael.
Within a line that stretches from Hor ha'Har in the north to Nachal Mitzrayim in the south (See also Na'ar Yonasan).
See Na'ar Yonasan.
I.e. the laws of Eretz Yisrael apply to ships within its borders (see Gitin 7b-8a). (PF)
This follows the opinion of those who explain that "u'Gevul" comes to include all that lies due west of Eretz Yisrael (See Na'ar Yonasan).