Waterproof

Dunno if you have a regular 12 or pro but this is what they say in the specs

iPhone 12 - Technical Specifications

• Rated IP68 (maximum depth of 6 meters up to 30 minutes) under IEC standard 60529

So more water resistant not water proof.

No iPhone is waterproof. Water resistant but not waterproof. Are you asking out if curiosity or are you planning a trip or activity around water and want to make sure you will be ok? Ultimately your goal should never be to intentionally submerge your phone but assuming nothing has caused a compromise point on your device it should be fine if it fell into water around the perimeters set for its water resistance shown in specifications

If you are going to be around water, my sister and brother in law have a boat, when I go out with them my phone spends the day in one of those waterproof bags they sell. Or failing that a ziploc (or whatever brand) baggie can serve to give your device some protection.