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Pretty much all iPhones have been for some time (since iPhone 7) to various degrees, but Apple themselves make no promises it will always work:

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

"Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear. Liquid damage is not covered under warranty, but you might have rights under consumer law."

I think of it more as good to have for an accidental drop, rather than a specification to rely upon for regular underwater photography, although the more recent phones do reach ever higher Ingress Protection (IP67, IP68 etc) water/splash/dust ratings.