My comparatively tiny marsh-y area experience was to put the matte medium down first. Then plant and touch up medium as needed.
As mentioned, probably difficult to work around the plantings after they are planted. Keeping the medium level and smooth, like undisturbed marsh water would be, was the challenge for me.
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(CD diorama competition)
More of a poorly drained vernal pond, it is hard to see under the mine supports. I added a lot of green "algae" like stuff and painted cattails on the ends of the grass
I experimented with clear Future floor finsh, as they do with model aircraft canopies, but it kept soaking into base layers and not creating a level surface. As a kid, I created a marshy diorama and used a piece of glass for the water surface. Not a terrible effect but hard to plant through the surface. Have toyed with the idea of thin, clear plastic that I could drill through the next attempt/experiment.
For that matter.... maybe paint a puddle of matte medium on smooth glass/plastic surface, let dry, peel, reverse for smooth side, and use as marsh surface? It is soft enough to poke holes or knife cut to add planting...? 2 cent brain fart... FWIW