The recently announced iPhone 7 may be water resistant, but don't expect Apple to replace it should you go swimming with it.
Previous generations iPhones, like the iPhone 6S and SE were "unofficially" waterproof, and it was expected Apple would make it official in this year's iPhone - which it did. It was kind of awkward for Apple not to announce it when their iPhones lasted longer underwater than other devices that should actually be waterproof, and let's not forget those that say you shouldn't use waterproof smartphones underwater.
And that's kind of what Apple is saying as well:
iPhone 7 and iPhone 7 Plus are splash, water, and dust resistant and were tested under controlled laboratory conditions with a rating of IP67 under IEC standard 60529. Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear. Do not attempt to charge a wet iPhone; refer to the user guide for cleaning and drying instructions. Liquid damage not covered under warranty.
So, you should consider you iPhone as able to withstand the ocasional and accidental dunk in the water, but don't treat like your friendly underwater camera when you go diving. In a best case scenario you may find it actually goes above and beyond (like that Apple Watch that spent 6 weeks underwater), but on the other hand... you may end up with an expensive reminder that smartphones and water still don't mix.