I was having massive battery issues on my series 0 watch (fully draining in under 6 hours! What?), but found this tip on Reddit that seems to be fixing the issue (although I haven't employed it long enough to know for sure my battery life will be fully back to normal):
Right after updating to watchOS 4 b2, my battery life was pretty crappy. I was getting maybe 5-6 hours. Did a clean install, didn't seem to help much, battery seemed to still be depleting quickly but I found a new feature setting - Detect Gym Equipment - that was turned on by default. It's in the watch app on the phone under the Workout setting.
Turned that off and battery seems back to normal now, at 87% after 6 hours off the charger.
Update: about 9 hours now off charger and at 75%