CCJAR, go look at the September 2008 Mapping Challenge - there are 30 entries and creating marshland/swamp was among the requirements on that mapping challenge. So you've got 30 examples on how to do it, each one slightly different so pick the one you prefer and ask for help.

If you're creating closeup swamps, that's more complicated but there are samples here as well.

I'm sure there's more answers than that to your question, but that's a start!

GP