Is iPhone XS Max waterproof?

The iPhone XS Max is water resistant, not waterproof. Please don't try to take photos under water. Apple will not cover repairs due to water damage and you'll have to pay the replacement cost out of pocket.

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

I wouldn't recommend it. See the following from iPhone XS - Technical Specifications - Apple

iPhone XS and iPhone XS

Max are splash, water, and dust resistant and were tested under

controlled laboratory conditions with a rating of IP68 under IEC

standard 60529 (maximum depth of 2 meters up to 30 minutes). 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.

Expanding on stedman1’s response:

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

There is NO model of iPhone that is waterproof.