what about wild cotton tree?
you can also use banana tree fiber unless that considered bark clothing and to advance for your people, seems like lousiana has banana tree when i google it (though i dont know is the species can be made into clothing or not). i assume this swamp is in more warm or tropical area right?
Hainan province is home to many ethnic groups; the Li and Miao people are particularly prominent on the island. The Li were the earliest settlers of Hainan. Their first-recorded appearance on the island has been traced back to 3,000 years ago.
The cultural heritage of the Li people includes many ancient and beautiful handicrafts, including the Li brocade, a unique textile product invented by the Li people.
Another source of pride of the Li people is a type of clothing: their extraordinary “banana clothes” are made from the bark of Musa Basjoo banana trees. Musa Basjoo banana tree bark is a source of natural fiber. The fiber has superior mechanical properties, high modulus, and tensile strength.
Banana clothes (芭蕉衣)
1. 1022
To make a banana clothing item, the Li people first peel off the dry bark of a wild Musa Basjoo banana tree.
2. The bark becomes soft after being boiled out and soaked for nearly half a month. The fiber comes out in strands by using a bamboo chip to scrape the softened tree bark.
3. Fiber yarn is then produced by connecting the strands end-to-end by the deft hands of a Li woman.
4. Banana clothes are made on a traditional weaving machine. The fiber yarns are dry and breakable and need to be moistened with water constantly while weaving the clothes.
5. The banana clothes are light, thin, breathable, and highly valuable; a piece of banana clothing takes three months of cooperative work by eight people.
also you may can try swamp milkweed, from google seems like it can be made into clothing too, for warm clothing, if your swamp is in cold region.
outside of plant base you can use animal leather or fur including reptile and frog skin.
also i think you can also try using spider silk if it exist as an option. 1022