The iPhone XS Max is not waterproof. Generally iPhones are built to withstand some water penetration but won’t always survive a drop in deep water or being submerged for an extended period of time. Read...some intense rain might be okay...the toilet, you might not be as lucky. Don’t go swimming with your iPhone! 🌊🚫.
Although the iPhone XS Max does have a level of dust and water resistance, Apple has noted that this resistance is not permanent and may decrease with time. Which also means that when you buy a refurbished phone you should take this into account. Depending on the age and repairs done on the handset, the device may have reduced resistance to water.
The water resistance might improve slightly with a case but the iPhone XS Max is not technically waterproof. You should always let your iPhone dry out completely before inserting any cables or trying to turn it back on.