@Mellon420, I appreciate all your trouble shooting tips, but I'm gonna push back a bit on your 1,000 cycles pontificating. My watch worked fine until a week and a half ago. At that point I had already been using my watch for 2 and a half years. I don't believe in coincidences. At that time I upgraded to 6.2.6. I used my watch normally on Saturday the 29th of May and around 3pm I got the pulse on my wrist that my watch was at 10%.
I have done all the trouble shooting of unpairing, resetting, your tips and calling Apple support. Apple did the battery diagnostic and told me the battery did not fail! They suggested giving it a couple months to see if it improves or to test the battery again. I can appreciate your advice on charging, but I'm no rookie to battery management as I drive a Tesla and keep it charged between 20 to 80% at nearly all times. It is impossible to do that with an Apple Watch, nor should you have to.
This latest update is causing battery issues for watch owners, there is nothing left to rule out. Maybe the update is the push over the edge for older watches, but there is no undoing the latest update. I wouldn't trust buying a new battery for my watch at this point anyway, so I will charge it twice a day until that becomes too annoying and I replace it.