Short answer: It depends on the maryadā (rules) you follow.

In Jain view, the main issue is:

• We avoid kund-mūl / underground edible parts, because uprooting them harms many living beings in the soil.

About water chestnut (singhāḍā):

1. Some Jain authorities say “Allowed (bhakṣya)”

- They consider water chestnut mainly as the seed/fruit of an aquatic plant, not a typical soil-root. - So they permit it for householders.

1. Other Jain authorities say “Not allowed”

- They treat it like an underground / tuber-type part and put it in the same group as potato, sweet potato, etc. - So they advise to avoid it, especially for stricter practice.

Because of this difference, the safest and proper way is:

• Follow the guidance of your own guru / local Jain samaj rules.
• If your family tradition avoids all doubtful underground items, then avoid water chestnut also.

You can see related answers here:

• Are water chestnuts bhakṣya?
• Can we eat water chestnuts?