I have modelled a marsh area. The area is grassy with three pools of what is supposed to be stagnant water. In the grass areas I have placed tussocks and the trimmed bristles from an old yard broom to represent reeds.
A boggy area I represent with a area which is made up of differing shades of grass.
"I would rather have a russet coated Captain who knows what he fights for and loves what he knows. Than he who calls himself a gentleman, and is but little else." Oliver Cromwell
I recently did some terrain pieces to represent Alaskan muskeg; I built bases with dark mud patches, dead/dying grass and dead trunks which can be used as-is to represent Boggy, and then smaller bases with stagnant water and vegetation on them can be added to turn it into Marsh. I think it works quite well.