I have worked for repair shop in the past and I have seen a lot of water damaged phones. In fact, a lot of these phones were actually marketed as being water resistant. Resistant being the key word, no phone is really guaranteed waterproof.
Usually the phones are supposed to work for around 30 minutes in one meter of water. Some are a little better, but if you read the fine print, you will invariably find that there is no guarantee that the phone will continue working after exposure to water.
In addition, water damage immediately voids your warranty, even if your phone was supposed to be water resistant.
I’ve included a list of water resistant phones, that I’ve personally seen, that have still been damaged by water. I’m sure there are more that I’m forgetting, but these are the most common:
iPhone 7 & iPhone 7+
Samsung Galaxy S5,S6, S6 Active, S6 Edge, S7, S7 Edge
Sony Xperia Z1, Z2, Z3, Z3 Compact, Z5
Some of these phones that are supposedly water resistant only got lightly splashed, yet still have been damaged by water. But I have also heard testimonials from people that have actually gone swimming and used their phone to take photos underwater. So what the deal? Why do some phones stop working and others continue?
Basically, it comes down to what condition the phone is in. A phone that was sold to you might actually have been waterpoof when you bought it. But general wear-and-tear will degrade the seals. In addition, a lot of these phones have flaps that cover up power ports or slots to ensure that water doesn’t get into the phone. These flaps are often easy to break off or lose. On top of all that, any cracks or damage to the screen or casing of your phone will also let water in. These cracks can be tiny, often people aren’t even aware of them before it’s too late.