Mine recently started chowing down on battery to the point that it was near death after 12 hours. A quick google turned up results indicating that certain apps permissions will drain the battery quickly. My plan was to reboot. If no better, I'd try removing permissions that didn't seem important (I have a vague recollection of being prompted for a permission the evening before the battery drain began). If that didn't work, I was going to reset it back to factory settings and re-customize where necessary, one step at a time. That's what my google search results suggested. Restarting didn't help - the next day, it was down to 15% by 6 pm after a full overnight charge. I don't know what permissions I removed the next morning - it was only one or two that looked unimportant - but I was back to using only about 45% in an average 18 hour day.

On the watch, go to settings -> apps -> permissions, and go through the various apps. I think I disabled location on s-health, but I may be wrong, or it might have been that and one other that I downgraded. Sorry, my brain fails me. The older I get, the better my forgetter gets.