I had this problem after wearing my watch while working outside on a hot day. Sweat had apparently gotten into the speaker holes on the side... This shorted out the watch and made it boot loop until the battery went dead.
I opened up the back of the watch and there were signs of sweat all inside it. After gently cleaning inside and all the little connectors with rubbing alcohol, I put it back together, charged it, and it worked like a champ again!
Clearly these things have vertualy NO water resistance...