So just a reply to help others since I myself had some difficulty.
1) As of 7/10/19, there's no WhatsApp app for Galaxy Watch.
2) The watch (both LTE and BT) can only receive notifications of a WhatsApp message and then can reply via text. This means you can't initiate a text to a whatsapp contact from your watch. You can only receive and then reply directly to that person.
3) WhatsApp calls from someone going to you does not work - you can't receive them on the watch. You do get notification of an attempted/missed call. Still can't call them back or receive the call from the watch. You also can't initiate a whatsapp call from your watch to another whatsapp user.
4) Missing notifications - this one took me awhile and after hours of support I thought this may help others. Speaking for myself with a LTE Galaxy Watch...I would not receive notifications from WhatsApp on my watch even though the watch had full LTE signal. Apparently the phone has to be powered up and have active Wi-Fi or LTE also. It appears the phone recieves the whatsapp notifications and then simply forwards them to your watch and to do so, requires the phone to be powered up and have connectivity also so it can talk to your watch. I thought when the phone was powered off or out of battery I'd continue to get notifications on my watch but that wasn't the case at all. So far, with the watch powered on at home and on Wi-Fi I can get notifications on my LTE watch while I'm out for a jog or bike ride.
Hope this helps others...
Galaxy LTE Watch paired to a Samsung S10+
I receive the WhatsApp message notifications properly except the WhatsApp call notifications, whatch doesn't vibrate when receiving the call on phone, after missed call from WhatsApp it shows a silent notification. How to get vibrate the watch when calling on the phone?
If it's any help I use a 3rd party app called Chat Hub. This not only shows you notifications from WhatsApp and Facebook Messenger, but lets you reply to these messages using your watch - provided that your watch and phone are within bluetooth range of each other (even if you have the LTE version). Just go to the Galaxy Store on your phone, search for Chat Hub, press Watch so it knows you're searching for watch apps. It isn't free but it was worth it for me for the small price (I took the risk), and although it only has 2 stars out of 5 under the reviews I can confirm it does work for me 100% of the time.
I bought my samsung galaxy watch active 2 days ago and it can’t give pop up on my watch when there is a whatsapp incoming call like regular phone call. I’m using Samsung Galaxy Note 8. However when my watch is pairing with iPhone it can perform properly and gives pop up too on my watch when I get whatsapp incoming call. How come it works with the iPhone but it does not work with his family Samsung. I’m so struggling about this and very dissapointed bought Samsung Galaxy Watch Active. Please share solutions anyone of you guys if you already found the solutions to fix it. Thank you so much. Cheers.