Thebes was the capital city of Egypt at that time. It was situated on the Nile at a place where the river spread out into a delta of many waterways. This meant that the city was surrounded by the river which gave it such a powerful natural defence that Nahum describes it as a sea and a wall.
Thebes also had some very powerful and loyal allies. Cush, her neighbour and the full resources of the broader nation of Egypt, would rush to her aid. The nation of Put and Libya, too, would side with Thebes if attacked. With such formidable defences and many allies to call on, Thebes was a very difficult city to defeat.
Thus, Nahum is asking Nineveh to compare herself to the formidable Thebes. Do they believe that they are better? That they have stronger defences that can withstand an invasion?
8 Are you better than Thebes that sat by the Nile, with water around her, her rampart a sea, and water her wall?