UPDATE 1:
As it turns out the stopwatch on the S21 goes up to 99 hours 59 minutes 59 seconds only. When it reached that I had to manually restart it, and the counter started from 0 again.
This many times was the stopwatch restarted: 4 times
Add 400 hours to the actual time that's on the counter.

UPDATE 2:
The phone gave a "Moisture detected" warning after being submerged for 117 hours and 53 minutes. The screen was unresponsive, and it kept jumping between applications uncontrollably.
After some button pressing now it works fine again, the moisture warning is still there.
Let's see what happens next.

Update 3:
At 241 hours and 10 minutes we were about a dozen people watching, and everyone was interested if the speakers still work. I played some music and the speakers sounded terrible, very quiet and barely understandable. At this time the water pressed "Lap", so that's what you can see on the screen now. It stays until the next stopwatch reset.

Update 4:
OBS (Streaming software) stopped working on day 15…