What is "Chavel ha'Yam"?
Targum Yonasan: Bi'Sfar Yama. Rashi - Chavel is a measure of land, which is measured via a rope. I.e. they are close to the sea. Who dwells there? "Kena'an Eretz Pelishtim." Pelishtim dwell on Yam ha'Acharon, in the west of Eretz Yisrael.
Radak: This is a portion, like "Chavel Nachalaso" (Devarim 32:9). It is Eretz Pelishtim, which is close to the sea - "Ad Yam Pelishtim" (Shemos 23:31). It says "Hoy", for their land will be destroyed.
What is "Goy Keresim"?
Rashi citing Targum Yonasan: A nation liable to be destroyed.
Rashi: It is a province of Pelishtim called Keresi, like it says about Tziklag "Anachnu Pashatnu Negev ha'Keresi v'Al Asher li'Yhudah" (Shmuel I, 30:14). Yonason translated there 'Al Deroma Kris', and it says there "b'Chol ha'Shalal ha'Gadol Asher Lakchu me'Eretz Pelishtim ume'Eretz Yehudah" (ibid. 16).
Radak, Malbim: Pelishtim are called Keresim - "Negev ha'Keresi", "v'Hichrati Es Negev Keresim" (Yechezkel 25:16). Malbim - they are called Keresim, for Yichresu (they will be cut off).
Why does it say "Devar Hashem Aleichem"?
Malbim: Hashem already spoke via Yo'el, Amos and Yeshayah, who were before Tzefanyah. Now it adds "v'Ha'avadticha me'Ein Yoshev."
Is "Kena'an Eretz Pelishtim"?
Radak: Yes - "la'Kena'ani Techashev Chameshes Sarnei Pelishtim" (Yehoshua 13:3).
