Is my iPhone 13 pro max waterproof

No, it is water resistant that's something entirely else as water proof. It can handle some water, but not all and water resistance wears off with time.

See this link for further reference

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

All iPhones begging from the iPhone 7 are water-RESISTANT. That means they will survive being sunk in water. But only for some time and not below certain depth. Your iPhone 13 Pro Max can be underwater UP TO 30 minutes and not below the depth of 6 metres.

mattiasfromczechia22 wrote:

All iPhones begging from the iPhone 7 are water-RESISTANT. That means they will survive being sunk in water. But only for some time and not below certain depth. Your iPhone 13 Pro Max can be underwater UP TO 30 minutes and not below the depth of 6 metres.

No, the iPhone has been tested under those specifications in laboratory conditions, there is no guarantee that after normal usage it will maintain those specs.

No. No phone is waterproof.