There's also quite the variation between northern and southern marshes. Most of the D&D marshes are the more stereotypical southern swamps with gators and outlandish critters.

One thing that is often true of southern swamps is that they are very fertile. You may find that the big difference you can refer to is jungle and irregular rivers cutting through wide silt fields.

AEG has a good d20 book on wilderness called "Wilds".

There is also a growing homebrew thread on swamps, again for D20, at Giant In the Playground.

http://www.giantitp.com/forums/showthread.php?t=72683