Is iPhone 12 Pro Max waterproof

The iPhone 12 Pro Max is water-resistant and not fully water-proof. You can find out more information about what this means at About splash, water and dust resistance of iPhone 7 and later – Apple Support

It is water resistant, not waterproof. See this:

https://support.apple.com/en-us/HT207043

No, there is no model of iPhone that is waterproof. While Apple markets it as “water resistant” there is no warranty coverage for a water damaged device.