Water Chestnuts, Canned - Ingredients Descriptions and Photos
From All-Creatures.org Vegan - Vegetarian Recipe Book: How Mary and Frank and Friends Eat

(Water Chestnuts, Canned) Water chestnuts are a wonderful addition to Chinese and Asian recipes, which add a crunchy texture to the dish. We only buy the ones that are canned in water with no additives. Unfortunately, during the canning process they lose some of  their sweetness and the layers of flavor that fresh water chestnuts possess. Since we don't live near to where this aquatic vegetable is grown, we only have access to the canned water chestnuts, but if you can buy the fresh water chestnuts, it is well worth the treat. What we eat as water chestnuts, are not really a type of nut, but the sliced root corms of the water chestnut plant which grow in the mud of the marsh, much in the same way that potatoes grow on dry land.  We could not find any detailed nutritional information, but some of the literature states that they are high in carbohydrates, and are also a good source of dietary fiber, riboflavin, vitamin B6, potassium, copper, and manganese.