Sadly I don't think the Apple Watch will get an app for WhatsApp even though it would be more convenient. At least there is quick reply which is handy but sometimes not always ideal as you can't a start a conversation with someone from your watch through WhatsApp.
WhatsApp updates are not so regular anymore which could either mean they are no longer supporting it as much or hopefully they are working on providing WhatsApp to new platforms.
Ive been waiting ages for this too. Obviously we can get message notifications and previews, but if WhatsApp get a good app going, it could be one of the best apps on the watch for messages. iMessage isn't too bad, and Facebook messenger is pretty bad on the watch. there is definitely a gap there of WhatsApp to fill!