Answer: Water chestnuts are not keto, as they contain an enormous amount of carbs.
Each serving of water chestnuts (1 cup) contains 24.2 g of net carbs. Sadly, that's far too carb-heavy to fit the keto diet.
For a vegetable with a similar crunch, try daikon radishes. At only 2.7 g of net carbs per serving, they're a far less starchy vegetable than water chestnuts
Also consider the delicious jicama. One of the few keto-friendly root vegetables, jicama are a similarly textured vegetable with a far lower carb count.