No solution to the problem, but only to share my experience with the 42mm version:
I've been draining 50% during a normal 12 hour day. That included a 7 mile hike without the iPhone where the steps, mileage, heart rate, etc all worked fine. Once it "found" my iPhone again, the Health Information all synced.
Even on the iPhone 6 Plus, I've gone through "battery saving tips" (e.g., I don't need my email checked every 5 seconds), which perhaps helps my Watch.
I'm still experimenting with the new Watch, but I haven't done anything "out of the box" and I'm using 50% battery per day.
Just FYI for comparison of another new Watch user.