Answer: Seltzer is a completely keto-friendly drink.
Seltzer, or "sparkling water," is totally compatible with keto. It's carb-free, and does not include anything that would interfere with ketosis.
Popular keto-friendly brands include:
Flavored seltzers are generally keto-friendly, but always check the label on an unknown brand to ensure it doesn't contain sugars or other keto-unfriendly ingredients.