iPhone X waterproof?

I definitely wouldn't. 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.

Only if you want to destroy it. The phones are not waterproof.

You might be able to get away with it. Once. Twice. Maybe three times. However, it's not advisable and if there's any damage Apple generally won't cover any damage under warranty. They have very specific definitions for what "water resistant" means, and it doesn't mean it's guaranteed to come away from submersion without damage.

Water resistant does NOT equal water proof. An IP67 or IP68 rating merely means a device is less likely to be damaged by accidental water exposure than a non-rated device. But an IP rating is not a guarantee that any water exposure may not cause damage.

About splash, water, and dust resistance of iPhone 7 and later - Apple Support