Unfortunately, Netflix appears to be down right now so I can't 100% confirm, but I believe it should work. I just tested with Amazon and it works. I can control volume, pause/play, and fast forward 10 or rewind 10. These are the same controls in the Amazon app. This is done by the player control that is inherent in iOS and not because of any app added. So if you can scroll up from the bottom of the screen on your iPhone and control Netflix, then you can control it from the watch.
You get to these controls via a glance on the watch. I keep this glance as one of the first in the list because I use it to control music on my iPhone when listening via headphones connected to my iPhone. If you are on the watch main view, you just scroll up and you are at the controller. Works great.