There have been numerous claims made that the iPhone 6s and iPhone 6s Plus are waterproof and that Apple has kept this feature a secret. There's a good reason why Apple doesn't publicized this.
When I heard about this I was skeptical. After all, Apple is not the sort of company to keep new features a secret, especially if the features give the new iPhone an edge over the competition. However, an examination by teardown specialists at iFixit revealed a new gasket around the frame and silicone seals around the connectors .
However, major water ingress points such as the speaker, headphone jack, and SIM card tray don't seem to have any waterproofing. There's also no waterproof coating on the actual circuit board.
So what does all this additional waterproofing mean in the real world? Testing seems to suggest that while the iPhone 6s and 6s Plus do seem able to survive a quick dip (although there are no guarantees), prolonged submersion will still kill them both.
The bottom line is that Apple didn't highlight waterproofing as a new feature because the iPhone 6s and iPhone 6s Plus aren't waterproof. They may be mildly water resistant, and may be able to survive a speedy dunk, if King Neptune gets a proper grip on your shiny new handset, it's history.
Keep in mind too that we don't know if there are any long-term issues associated with even short periods of immersion.
Also, survival will vary depending on the sort of water it falls into. Clean tap water gives it a great chance of survival than corrosive sea water. So bear that in mind too.