There is something in many cell phones that indicates if it has been submerged. It's some kind of reactive film or wire. I guess its like a fuse. Once its gone, it needs to be replaced. It could be some sort of memory or electric shock protection. From what I understand its also used by companies to determine cause of cell phone death. Might also be used to determine if a phone had been tampered with.

Sometimes moisture may not get to this film/fuse.

There is a very limited window from when a cell phone gets wet/submerged to when it cannot be repaired. This is due to rapid corrosion of some critical parts.