I had my iPhone 6P for almost six months before I bought my AW Sport. I use the notifications from iMessages, the Calendar App, a Weather App and I occasionally take phone calls. There is no FaceTime camera on the Watch, so this is not a factor. I also use it for the Workout App, Pedometer App and United Airlines App.
I did not notice any additional drain on my iPhone's battery when I started using my new Watch a year ago. The BlueTooth 4.0 the Watch uses to communicate with your phone when it is in range is very low power. When you are out of BT range and WiFi kicks in, the amount of traffic to and from the Watch is not enough to make a difference with your phone's battery usage.
TxWatch