Musty swamps are full of unique creatures and funny plants like cattails. Water soaks most of the soil and these environments offer a good mix of deep water with shallow ground.
Concept artists have designed a wide variety of swamps and marshes for video games. These are still popular environments and they’re great practice for an environment art portfolio.
Check out these examples for some high-quality concept art environment inspiration. Marshlands may be tricky to design without reference, but it’s helpful to study what others have done and add your own ideas to get some unique paintings.