Can seaplanes land on normal airfields or in a marsh or muddy area?
Back in the early 1970s I belonged to a Civil Air Patrol (CAP) squadron that had a J-3 cub with an 85 horse engine on it. During the summer we would operate it on floats, during the winter on wheels. At the end of summer, we would land it on the grass alongside the runway at the Springfield, OR airport (no longer there, it's a big Walmart now) and replace the floats with wheels. In late spring, we'd replace the wheels with floats and take off from the grass.

The joke was that for takeoff you were supposed to line up all the cadets and have them pee in unison along the path the floats would take to aid in the takeoff. Actually, that wasn't necessary since western Oregon gets a lot of rain. Even with wet grass though, for the takeoff we'd get the initial movement by pushing, and we were always careful to ensure that the grass had nothing on the takeoff path that would damage the floats.

With the floats on, you could fly when the airspeed said 40 mph (yes, it was indicated in mph) which is about 35 knots. You could generally count on at least 5 knots of wind right down the runway when it was raining, so at 30 knots across the grass you were good to go.

You can go to YouTube and search "floatplane landings on grass" to see some examples of landing on grass. "Floatplane takeoffs on grass" will show some takeoffs, but a quick look showed using a dolly, which I think we did once or twice before my time with the group but then figured it was easier to just use the grass.