For most of history, water transport has been more efficient than land transport. We are so used to good roads and before that, trains, that we think of that type of movement as the default human strategy. Water transport can certainly facilitate empire, but nationalism, the uniting of closely-related groups that are within a physical area, seems to have grown up just after slightly better roads, and then trains. I suppose canals should be included, as they are water transport, but internal, and bear a lot of similarity to land transport.
When you are trading by water you have city-states. When others are accessible by land we start to feel more connected to them. I'm sure historians have been noticing this and writing about it for years and I am just now picking up on it.