There have been other questions that have touched on the leather thing, like this one. Pretty much, the biggest problem for an aquatic nation is that you actually need land and drying time in order to make leather. You don't have the hurtle, you are just using the land/water as a resource.
Amphibian is a weird word, because it can have different definitions depending on who you ask. Scientifically, we're talking frogs, newts, salamanders, that kind of thing. Historically, according to the Greeks, things that lived in and out of the water, like, say otters and your crocodiles.
While the Chinese giant salamander is large enough to make leather from on a larger scale, I can't find any evidence that anyone ever actually has. There's a Salamander Bonded Leather Company, but that seems to only be the name, and so salamanders aren't actually involved. However, don't let that stop you from doing so in your fictional world. The environment they live in sounds more like mountain streams than it does swampland, so it may not fit what you are doing.
However, when it comes to toads and frogs, there actually are places selling that.
The main problem is size, because you'd have to piece together small frog "pelts" to even make an outfit.
The answer to that is either giant frogs, which I fully endorse, or a type of bonded leather, where leather is:
made by shredding leather scraps and leather fiber, then mixing it with bonding materials. The mixture is next extruded onto a fiber cloth, or paper backing, and the surface is usually embossed with a leather-like texture or grain. Color and patterning, if any, are a surface treatment that does not penetrate like a dyeing process would. The natural leather fiber content of bonded leather varies. The manufacturing process is somewhat similar to the production of paper. SOURCE
Or you can just have them sew it all together.
AS far as thread and fabric is concerned, since you are building a world, and although this isn't what you asked for (since it is more sea life, though it could show up in a swamp estuary), there's byssal threads from mussels.
Now, looking at plants, the thing is that tech level is very important as far as answering this. Today's list of plants that can be made into fabrics is extensive, and many were unthinkable just 20 years ago. Reeds, straw, and bamboo can be made into fabrics. Really, most are just converted into rayon.
Give me a tech level, and I can start to answer as far as plants are concerned. With the right manufacturing, lots of stuff can be used that way. But if it's low tech, then you have a problem.
There's good news though. If you aren't married to it being a plant that grows IN the water, flax may be surprisingly your best bet.
Flax is often found growing just above the waterline in cranberry
bogs. Heavy clays are unsuitable, as are soils of a gravelly or dry
sandy nature. Farming flax requires few fertilizers or pesticides.
Within eight weeks of sowing, the plant can reach 10–15 cm (3.9–5.9
in) in height and grows several centimeters per day under its optimal
growth conditions, reaching 70–80 cm (28–31 in) within 50 days.