When should they sing a new song to Hashem?
Radak: At the time of Yisrael's redemption.
Malbim: The new wonders will be unlike the old wonders. Before, Hashem's honor was revealed amidst the settled land, in Egypt and in Eretz Yisrael, and from their reports spread to the entire world. In the future, the miracles will be at once at the end of the land. His praise will come from the end to the middle, and from Midbaros and seas to civilization. There was no song like this until now.
What is the meaning of "Tehilaso mi'Ketzei ha'Aretz"?
Rashi: When they see My Gevurah for Yisrael, all the Nochrim will admit that I am Elokim, Bal Korcham (against their will).
Radak: The Goyim at the end of the land will tell His praise. "Mi'Mizrach Shemesh Ad Mevo'o" (Tehilim 113:3) will know about the Ge'ulah of Yisrael. They are scattered in all corners. When the Goyim see those wonders, they will sing His praise.
Malbim: Refer to 42:10:1:2.
Who are "Yoredei ha'Yam"?
Rashi: They are people who embark on boats. Radak - i.e. both at sea and on the land they will sing His praise.
What is "u'Melo'o"?
Rashi: These are [houses] fixed in the sea, and not on islands. Rather, amidst the water, everyone pours water, enough [to make a place] for a house, and they go from house to house via boats, like the city of Venice.
Radak: This refers to fish, which fill the sea. This is a metaphor, like "v'Chol Atzei ha'Sadeh Yimcha'u Chaf" (55:12).
Why does it mention islands and their residents?
Malbim: Even though they are totally separated from civilization [also they will tell His praise].
