I ran a one-session swamp crawl a few years ago. In places it was shallow enough to walk, but they generally went by raft. All kinds of nasty things hide under the surface - they encountered giant leeches when they stopped to investigate a tunnel of water suspended by magic.
The town they encountered was built around and on top of a spire of rock, with cranes and pullies for transporting boats up to the lowest levels.
Have you run anything cool in a swamp setting?