Water damage. So many posts about people taking their "water resistant" phones under water, in the ocean, on a water slide, etc. The phone is NOT "waterproof"... it's water resistant, and Samsung has stipulations in their manual about how this phone can and cannot be treated. The IP68 baloney rating is misleading, not to mention all the stupid Youtube videos showing it's alright to soak the phone... and that's why people keep doing it. Even "running tap water" is a no-no according to the Samsung manual.
The only thing you can do is stick the phone in some rice, or some other desiccant, and leave it there for a day or more... then hope for the best. If the phone doesn't recover, Samsung will probably not even cover the phone repair... unless you're extremely lucky.