Went for a sea swim on Monday wearing my 3-4 year old 5S which I have done quite a few times with no issue.
Watch was tracking time ok for at least first 20mins - I didn't look at my watch again until I finished and went to stop tracking and the screen was black as if battery had run out. This could have been possible as it was quite low when I started.
Plugged in to mains charger at home & the watch vibrated as normal. I left it a while & when I came back to check on it the watch was super hot! And the screen completely black. Unable to turn on.
Watch was a Christmas present 2019 so I don't have the receipt & am unsure how long any warranty lasts!
