Short answer: There is some difference of opinion, so it depends on which guidance you follow.

In Jain view:

• Water chestnut (singhāra) grows in water. Botanically it is the seed/fruit of an aquatic plant, not a normal underground root like potato, onion, garlic, etc.
• Because of that, some Jain scholars and families allow it (consider it bhakshya) and eat singhāra flour (singhāra atta) in normal days or on certain fasts.
• Others treat it like an underground / tuber‑type item and avoid it, saying it still involves more himsā and should be left for safety.

So:

• If you follow very strict ahimsā with no doubtful items, better to avoid water chestnut.
• If your Guru, sadhu/sadhviji, or family tradition clearly allows it, then you may eat it as per that maryādā.
• In Jain Dharma, when there is doubt about an item, we usually follow: “sandeh hoy to tyāg śreṣṭh” – in doubt, renunciation is safer.

For your personal practice, the most correct way is to follow the guidance of your own tradition / local Jain sangh.