I've been fiddling around a long time trying to make textures that look like marshes or swamps, but most have never been very successful.
I did hit on another way to make a good simulation of a marsh by using water as RGB 30 and then assigning a texture with predominately green/brown coloring. At the same time, my_mapC (which assigns where water appears through an app that creates map_c) needs the marsh areas to be given RGB vales of around RGB 120 to 130.
Like this:
Normally these light RGB values create surf or underwater reefs/coral/shallows, but if it is covered by RGB 30 in map_t, then the surf disappears and the texture assigned to the water RGB 30 slot in the load.ini will become very predominant and 'shine through' giving the water a greenish marshy effect. Of course care must be taken to use a suitable texture. Here is the texture I used in the examples below (not to scale).
Here are a few screen shots to show the effect:
Of course not many maps need marshes, though you'd be surprised how often you find them in coastal desert regions.