I have not updated my Samsung Galaxy Watch in over a month (it says no update is available). About 3 days ago I started noticing my watch was dead about halfway through the day. I changed nothing out of the ordinary and am unsure as to what could have caused this. I reset everything. I tried turning everything off to "conserve" battery although i think that is counter intuitive to why I bought the watch in the first place, but just to test it I did so. Nothing helps......the battery last 5 hours if I don't use it at all and less than an hour if I am using it for a workout or anything. I have previously used my watch during my runs for over an hour, and still had 75% battery left at 8pm after having it on all day. This is devastating because I truly loved my watch.
Several people have posted that it is tied to the gear app forcing a constant wake from your phone. Try removing the app / clear cache and reinstall the application and then add watch to phone and see if that clears it up