Many people think that if they want to soak in a relaxing bathtub, they have an iPhone at hand. Certainly, listening to music and watching movies with the iPhone will not be boring. You can avoid situations where you miss a precious phone call in a long bath.
In conclusion, you should avoid using your iPhone in the bathroom. Recent iPhones have high splash / water / dust proof performance, the latest model iPhone 11 Pro / Pro Max can withstand a depth of 4m / up to 30 minutes, iPhone 11 can withstand a depth of 2m / up to 30 minutes Therefore, it does not break down when it is slightly wet, but considering the following, you can think of use in the bathroom as NG.
First, in the international standard on waterproof performance (IEC standard 60529), tests are performed at normal temperature / fresh water. The water in the bathtub or shower is around 40 degrees, so it is obviously not at room temperature. Hot water mixed with soap and shampoo bubbles and hot water containing bath salts are likely to adhere, so it cannot be said to be fresh water.
Another reason is that the waterproof performance is not taken into account when connecting to other devices / cables such as charging. If you submerge your iPhone, the Lightning port will be wet and will not be able to charge safely. Apple's published document on splashproof, water and dustproof performance of iPhone 7 and later (LinkAlso states that "Charging with a Lightning cable or connecting a Lightning accessory should take at least 5 hours."
What is decisive is that damage due to water exposure is not covered by the warranty. The iPhone has a built-in liquid ingress indicator to determine if it has ever been in contact with water or other liquids, so you can spot it if you don't use it in the bathroom. Even if you use it in the bathroom after preparing for the breakdown, you have to bear the cost.