I got this same problem in my Samsung Gear Sport watch (bought 4yrs ago, no warranty) after swimming and pressing the buttons inside the pool. The next moment it went into rebooting loop; I wiped the watch and coved the watch inside a bag of rice (for dehrdatrion) for a week. This removed all the water drops inside the watch. After a week I charged it and, my watch is back to normal. My observation is these watches are actually not waterproof after 1 or 2 years, so water goes inside and cause this rebooting loop. Just need to remove water completely. Hope this helps someone.
i also got similar problem, i solved it by teardowns the watch (i follow this instruction ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-5DRNFWJ5fxQ.htmlsi=yBQ_YiR41TXt4Ln3), then use heat gun to dry every part for 1-2 minutes (except battery) after that my watch work perfectly fine.