Jamieson, Fausset and Brown on Isaiah 33:21

21. there--namely, in Jerusalem.

will be ... rivers--Jehovah will be as a broad river surrounding our city (compare Is 19:6; Na 3:8), and this, too, a river of such a kind as no ship of war can pass (compare Is 26:1). Jerusalem had not the advantage of a river; Jehovah will be as one to it, affording all the advantages, without any of the disadvantages of one.

galley with oars--war vessels of a long shape, and propelled by oars; merchant vessels were broader and carried sail.

gallant--same Hebrew word as for "glorious," previously; "mighty" will suit both places; a ship of war is meant. No "mighty vessel" will dare to pass where the "mighty Lord" stands as our defense.