Jamieson, Fausset and Brown on Isaiah 42:10

10. new song--such as has never before been sung, called for by a new manifestation of God's grace, to express which no hymn for former mercies would be appropriate. The new song shall be sung when the Lord shall reign in Jerusalem, and all "nations shall flow unto it" (Is 2:2; 26:1; Re 5:9; 14:3).

ye that go down to the sea--whose conversion will be the means of diffusing the Gospel to distant lands.

all ... therein--all the living creatures that fill the sea (Psa 96:11) [Maurer]. Or, all sailors and voyagers [Gesenius]. But these were already mentioned in the previous clause: there he called on all who go upon the sea; in this clause all animals in the sea; so in Is 42:11, he calls on the inanimate wilderness to lift up its voice. External nature shall be so renovated as to be in unison with the moral renovation.