There are probably detail parts in plastic or brass for things like the weeds, cattails, and bullrushes. However, the standard scenery materials will probably do... Twine, natural materials, ground foam, an so on.
For areas of open water, there are a variety of products. I did a small wetland diorama (HO scale) this summer to try things. I painted the open water areas black and covered them with gloss meduim. While it was still wet, I sprinkled in some ground foam to represent duckweed (the little floating leaves). Looked pretty good.
The raised areas I built up with sand, soaked in 50/50 water/glue, and then layered with ground foam, crushed leaves, and other landscape materials.
Good luck with your project!
Andrew
