The guys gather at the river's edge and swim naked across to a home apparently rented by a bunch of French women. The river is a natural break between the land of brutality and gunshots and the land of…well, good stuff. Rivers are important in the world of literature. We sit up and pay attention whenever one appears, mostly because one of the most famous rivers of all time has to do with death – that is, the River Styx. In Greek mythology, the River Styx formed the border between the land of the living and the land of the dead. Lots of mythological figures got into trouble trying to swim from one shore to another. We could think of this river as the separating Paul and his friends from a world of peace and love. The river is an actual boundary, remember? The soldiers aren't legally allowed to cross it, and so they have to sneak across at night.
We also think it is fascinating that this river happens to be the first real body of water we are exposed to in this story. Water is scarce, and that makes sense to us. When we think about water, we think about things like plants, grass, life, slip n' slides, and things that grow. The war that Paul observes is definitely not about such fun things.