I know there are some amphibious planes which can land on both water and land but how different is the their landing in comparison to that of a normal small aircraft?
Also, if seaplanes needs to go into some marsh, will that work for them?
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.
A pure seaplane cannot land on a normal (land) airfield: The hull or floats would be badly damaged.
An amphibious aircraft can land on both water or a normal airfield (pavement, grass, gravel, etc.), because it has both wheels and floats, but with amphibious aircraft you're either operating as a seaplane (on water) or a landplane (on solid ground), and you would follow procedures appropriate to the type of operation you're conducting.
As I understand it amphibious aircraft would not be suited to landing in a "marsh" or "swamp": The water and mud would render the wheels ineffective (and possibly dangerous: If the wheels dig in to the mud the aircraft could flip over), and the dirt/land portions of the area would damage or destroy the floats/hull.
It may be possible to put the aircraft down in such an area, but it would be very unsafe, bordering on reckless, to do so in a non-emergency situation.
Seaplanes can land on land (pun intended) if the ground has a soft cover. This could be snow, tall grass and landing in a wet peat bog is also possible. However, any solid object close to the surface (logs, stones) will damage the hull.
Historic cases are the landings of Amundsen with a Dornier Wal on arctic ice or the landing of Gronau's Wal when it was transferred to the Museum in Munich (actually, this was the same aircraft). It was landed in winter on a snow-covered meadow close to the Isar river in central Munich.
This was in no small part possible due to the flat hull shape of Dornier flying boats. They were optimized with water tunnel studies for the heavy and inefficient engines of their time. Modern hulls have a keeled shape, where the bottom is v-shaped to reduce the load factor when planing in rough sea. Landing such a seaplane on land needs more snow and should not be attempted on grass.