Nahum asks this rhetorical question to emphasize the negative answer that it anticipates. AT: "you are not better than Thebes ... itself." (See: [[rc://en/ta/man/translate/figs-rquestion]])
These two phrases share similar meanings. The words "ocean" and "sea" both refer to the Nile River, which ran near the city. Nahum speaks of the Nile as if it were the wall that protected the city. AT: "which had the Nile river as its defenses, as some cities have a wall for theirs" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-parallelism]])