A new app, WristChat, enables Apple Watch users to respond to messages from the device. You can reply to a message using Scribble, Voice Dictation, or the custom-built WristBoard, a keyboard designed for the smaller screen with swipe typing and autocorrect. There are many other customizable features such as haptic feedback and chat bubble colors.
https://www.notebookcheck.net/Apple-Watch-users-can-now-reply-to-WhatsApp-messages-on-the-device-thanks-to-WristChat.613368.0.html