Is the iPhone 11 waterproof?

They are water resistant, which is different from waterproof. Means that if intact, an occasional splash or dip remediated promptly will probably not cause harm. In standards terminology and in the case of the iPhone 11, it complies with IEC Standard 60529 for IP68 water resistance. This translates to being able to resist a dip up to 6 meters for up to 30 minutes without harm, when new and is undamaged.

If the seals have aged, if it has been altered or opened by unqualified personnel, if the screen or back is cracked, etc., then all bets are off and it will die as soon as wet. And any internal water damage voids the warranty.

There is no model of iPhone that is waterproof.