It's hard to imagine a time when iPhone models were not waterproof but there was. Apple released the first "water-resistant" iPhone in 2016, and now all Apple products are listed as "H2OK" aka waterproof. The iPhone 12 mini is no exception coming with a rating of IP68 under IEC standard 60529. This means the mini is splash, water, and dust resistant at a maximum depth of 6 meters up to 30 minutes. Resistance to water damage might decrease with time.
Remember to never charge a wet device, and that liquid damage is not covered under Apple's warranty.