iFixit
But it turns out that $4, as well as silicone seals around important components, according to $4.
Both the gaskets and seals appear to be designed to keep water out.
YouTube user Zach Straley put the $4 to see what would happen.
The iPhones emerged as if nothing happened, but a $4 after the test showed a diagonal line appeared across the iPhone 6s' screen.
A different YouTube video from iDeviceHelp put the new $4. The screen started to fade after about two minutes, and then simply died.
So 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.