If you want to use your smartwatch with a custom launcher, you will find out that there is no way to get access to notification bar. Then you need to install some apps to get access to them.
I found this app very useful and well made. It will show a floating notification on your screen (you can decide the position) and tap on it to open the relative app. The lite version will not allow you much more than that (but still, it’s enough to solve our main problem).
The paid version unlocks some very nice features. You can customize the action on your notification (swipe, double tap etc) to expand, hide, close, even quick reply on your notification. For example, if you swipe down on a Whatsapp notification, the keyboard will show up and you can answer without opening the app. Also you can set the screen to wake up on notification and you have lots of nice things to customize.