R’ Chaim Kanievsky once asked the Chazon Ish, that since the Mediterranean is the border of Eretz Yisroel (see Rambam, Hilchos Terumas 1:7) how is one allowed to swim there, surely, he is violating the prohibition of leaving Eretz Yisroel to go to Chutz La’aretz?
The Chazon Ish answered: The areas which are suitable for bathing and swimming in, are still considered Eretz Yisroel (Kuntros HaZichronos, 46:2). R’ Chaim Kanievsky explains: Since for those who live nearby the area is suitable for bathing and washing in, it is considered as part of the dry land near to the sea (Orchos Rabbeinu, Vol. 3, pg. 198).
R' Yitzchok Zilberstein brings proof to this from the Gemara in Eruvin (61a) and the Shulchan Aruch (398:9). We find a halachah that if there is a nachal [stream] find by the side of a city, if the people of the city use it it’s nullified to the city, and we measure 2,000 amos for techum Shabbos from the end of the nachal. Therefore, here as well, the part of the Mediterranean that people use is nullified to the dry land nearby and has the status of Eretz Yisroel.
