Is iPhone X waterproof?

NO, on Apple.com iPhone is listed as Water Resistant, NOT WATERPROOF. If you got your phone wet and it is damaged because it got wet, it is NOT covered under Apple Warranty. You'll have to pay for an out of warranty replacement or if you have AppleCare+ the replacement price is reduced.

The specs say water resistant, not waterproof. See the following from iPhone X - Technical Specifications

iPhone X is splash, water, and dust resistant and was tested under controlled laboratory conditions with a rating of IP67 under IEC standard 60529. Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear. Do not attempt to charge a wet iPhone; refer to the user guide for cleaning and drying instructions. Liquid damage not covered under warranty.