The experiment and the stream ended at 745 hours, which is 31 days. This is the follow up time lapse video.
Samsung Galaxy S21 kept underwater for 31 days.

I got my SAMSUNG GALAXY S21 5G on January 29th (launch day) and I decided to put it in an aquarium of water to stream the event until it breaks. I can't be the only one who is curious :)
I taped a wireless charger taped to the back of the fish tank, that kept the phone fully charged.
The Samsung Galaxy S21 has an IP rating of IP68 which means it's resistant to submersion up to a maximum depth of 1.5m underwater for up to thirty minutes.
Samsung Galaxy S21 5G water test. Keeping a Samsung Galaxy S21 underwater test.

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: 7 times
Add 700 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 and the stream was restarted.

Update 5:
At 351 hours 9 minutes a couple of new notifications appeared on the notification bar. I had to investigate and this is the photo I took: https://imgur.com/UJ1uDTx (backup url: https://ibb.co/Zczqr52)
These are most likely not related to the water at all. (Note: the phone is not connected to the internet)

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***Disclaimer: This video is for entertainment purposes only. I take no responsibilty for any damage you cause to your electronic devices. I take no responsibility for your or anyone's health if you injure yourself or others while tryting to find out if your phone is also waterproof.
Electronic devices, electricity and water are not a good match, they can cause serious or even fatal injuries.