According to the interwebs they are the the ultimate survival food. https://plants.usda.gov/plantguide/pdf/cs_tyla.pdf says that one can make 0.75 kg of flour from each square meter of cattails. This flour has 266 calories per 100 grams or 2000 calories per 750g.
I’m no farmer, but making flour from starch should just evaporate the water, leaving the nutritious stuff behind. Therefore, each farmed tile of cattail should yield 7-8 rhizomes.