A water chestnut is part of the root, or corm, of a grass-like sedge. The blessing that should be recited before eating it would therefore be "Borei pri ha'adama" as this is considered a vegetable.

These should not be confused with the chestnuts that grow on trees, and require the Ha'etz (fruit) blessing.

Enjoy!

Please be sure to see our Brachot Wizard.

All the best,

Rabbi Shmuel Kogan,
Chabad.org