I learned the hard way also that IP68 may failed on S21 ultra. Previously, I took a few of shallow pool underwater pics with it and nothing happened other than temporary muffled speaker and wet charging port warning.
But this time, trying not to wet the charging port and speaker, I dip HALF of the phone underwater in a hotel pool for a few seconds to take picture and the screen went greenish and camera failed warning shown. I immediately pulled it out and tried to turn it off, then my phone died.
I left it for a few hours while driving home and took it apart, hairdry each components, put the components in a box of rice and later with a box of silica gel overnight. I started to look for replacement parts such as screen, cameras and speakers.
Luckily the next day when I put it together and plug the charger the screen is fine, but it refuses to charge (grey circle with lightning). Press the volume down and power several times to reset it, then it finally started to charge. I then turned the phone on, the screen stills flickers a bit from the water damage but the phone works but with no touchscreen and fingerprint reader. I unlocked the screen with samsung's find my mobile (google find my phone can't do that) and I managed to get the phone connected to my wifi, allowing foldersync to run its job backing up my phone which I set before. With my data safe and sound, I took the components out again, cleaned it throughly and dry it again in the box with silica gel.
I thought I'd have to replace the screen but the next morning turned out the touchscreen and fingerprint reader worked just fine. Maybe I was just lucky, but I think in any water damage, time is critical to immediately turn the phone off and took the components apart for drying. It is also important to give some time until the components are throughly dry. Don't rush to buy components as they maybe durable enough to survive.