iFixitApple didn't say anything about extra water resistance in the new iPhones.
But it turns out that Apple added a gasket around the new iPhones' frame, as well as silicone seals around important components, according to iFixit.
Both the gaskets and seals appear to be designed to keep water out.
YouTube user Zach Straley put the iPhone 6s and the iPhone 6s Plus in large, shallow bowls full of water for an hour to see what would happen.
The iPhones emerged as if nothing happened, but a follow up video of how the phones fared 48 hours after the test showed a diagonal line appeared across the iPhone 6s' screen.
A different YouTube video from iDeviceHelp put the new iPhone 6s Plus under four feet of water. The screen started to fade after about two minutes, and then simply died.
iDeveiceHelp/YouTubeSo it seems that Apple's waterproofing measures for the new iPhones were never designed for extended water submersion or swimming. But the measures do help with some real-life situations where your iPhone could become damaged from water-based accidents, like dropping in toilets.
A word of caution: Just because Apple made the iPhone 6S more resistant to water, doesn't mean you should go out of your way to submerge it.