Source Gittin 8a which is regarding what part of the mediteranien sea is obligated in Maaser (see 7b-8a) (Bold is translation everything else is Research)
הנסין שבים רואין אותן כאילו חוט מתוח עליהם מטורי אמנון עד נחל מצרים מן החוט ולפנים א"י מן החוט ולחוץ חו"ל רבי יהודה אומר כל שכנגד ארץ ישראל הרי הוא כא"י
The Islands in the Mediteranean sea within the imaginary line between the Turei Amnon (or Hor Hahar mountain as Rashi Translates)
{ - According to Rav Estori Haparchi (14th Century Traveller written in Kaftor vapherach) brought down in LÀ GÉOGRAPHIE DU TALMUD (press ctrl-f search "endroit du nom de kaplaria if you understand french) by Adolphe neubauer, this is Jebel Akrra which has many ancient sites identified with Hitites part of the ancient Canaanites by archeoligists of Mount Kilic see reviews below Antioch (antakya in modern day turkey) where the border of Israel is supposed to pass below, as the Targum Yerushalmi Translates Numbers 34,8: מֵהֹ֣ר הָהָ֔ר תְּתָא֖וּ לְבֹ֣א חֲמָ֑ת as מִטַוְורוֹס מָנוֹס תְּכַוְונוּן לְכוֹן לְמַעֲלֵי אַנְטוּכְיָיא וִיהוֹן מַפְקָנוֹי תְּחוּמָא. This mountain slopes right into the sea.
- According to Historian Dean Philip Bechard in Paul Outside the Walls: A Study of Luke's Socio-geographical Universalism in Acts 14:8-20 Turei Amnon (gemora)= Turus amunus (the Targum Yerushalmi could also be refering to this as טַוְורוֹס מָנוֹס), a range of Mountains that reach from the Northern medetaranean till the Euphrates river just north east of Antakya even further north then Mount Kilic.
- Alternatively according to Rashi Makkos 9b who says the land is divided into 4 equal parts from north to south the 3rd of which is Shechem (Nablus) to Chevron which is 49 miles, Hor Hahar (98 miles north of Shechem) could be identified with Jabal Baiquon which slopes down to the town of Remaile on the cost just north of Sidon}
till the valley of Mitzrayim
{ - Most seem to think this is Wadi el Arish like Rabbi Saadia Gaon (in fact he lived in Arish).
- Rashi seems to think its the Nile in Joshua 13,3 (Rambam in Cairo could have been living in Israel on the Eastern front of the Nile)}
are part of Israel to obligate Maaser. Rabbi Yehuda says parralel lines from top and bottommost border of Israel till the Ocean i.e the islands (i.e not mainland see Tosfos דלא איירי ביבשה שעד אוקיינוס אלא בנסין שבים הגדול דווקא) require Maaser.
Bearing in mind Tosfos quoted says only Islands are included not sea bed or mainland (It dseems if the islands expanded more would be part of Israel but if mainland e.g Africa expanded this would not be part od Israel, These Islands seem to be Chayav Maaser according to both opinions of Rashi and Rav Ishtori Haparchi are part of Israel: Zireh Island (Moder day Lebanon next to Sidon), Adam rock off Bat yam beach, Rosh Hanikra Islands and Achziv Islands near the border of Israel with Lebanon.
If the line goes from Mount killic (Rav Istori Haparchi opinion) then also the inhabited island Arwad, uninhabited Al Abbas, Jasirat basira, islands of Abu al faris (all part of modern day Syria), the Palm Islands nature reserve (includin 9 islands next to Trippoli, Lebanon) are within the line according to Chachamim and also Rabbi Yehuda
If Hor Hahar is as far north as mount kilic in Turkey or the Taurus Amunus range, Major Islands of Cyprus, Crete (Greece) and Sicily (Italy), Malta among many others are parrallel to Eretz Yisrael
and are part of it to require Maaser according to Rabbi Yehuda. But even if Hor hahar is lower down like near Sidon the island of Djerba (Tunisia) is certainly aligned with Israel according to Rabbi Yehuda (but not Rabbannan) see map