Despite what you might have read, the iPhone 7 is not waterproof.
It's not like me to be pedantic about things -- typically I don't have a problem with simplifying things for the sake of clarity -- but this is a time when precision is important because there's a huge difference between a device being waterproof and being water-resistant.
To understand the difference, take a waterproof raincoat. You'd be right to expect to stay dry in a rainstorm wearing this jacket -- given that it's a raincoat -- but if I were to blast you with a fire hose, it's likely that water would make its way through both the fabric and the seams (and it might very well tear the seams). And that's an extreme example, since even a heavy thunderstorm or a tropical rainstorm might be too much for some raincoats.
So a raincoat is waterproof, but only in the context for what might be considered normal rain. Outside of that and all bets are off.
And that's the problem with the word "waterproof." It's not a defined quantity. Which is why electronic devices are said to be "water-resistant." And this term comes with definitions attached.
And this is where it becomes important to read the small print.
Here's Apple small print for the iPhone 7:
"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."
Let's break this down.
"iPhone 7 and iPhone 7 Plus are splash, water, and dust resistant ..."
Not waterproof. Not dustproof.
"... tested under controlled laboratory conditions..."
Not under real-world conditions, and not in some test conjured up by someone making a YouTube video. This is a fancy way of saying "your mileage may vary."
"... a rating of IP67 under IEC standard 60529."
OK, there's a lot going on here. IP stands for "Ingress Protection," and this is defined in the international standard IEC 60529. IP ratings consist of two digits; the first shows the level of protection against solids, and the second for liquids. IP67 means that the iPhone 7 is totally dust-resistant (the first digit), and capable of surviving temporary immersion up to 1 meter for 30 minutes in pure water (not cola, not sea water, not the bleach in a toilet).
Samsung's Galaxy S7 had an IP rating of 68. This, according to Samsung, means that on top of being totally dust-resistant, it is water-resistant in up to 5 feet of water for up to 30 minutes. Technically, IP68 means a device is rated for continuous immersion, but it seems Samsung wants to err on the side of caution. Additionally, the Galaxy S7 has passed MIL-STD-810G testing against a subset of 20 specific environmental conditions, including temperature, dust and sand, shock and vibration, low pressure, and high altitude.
"Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear."
Worth bearing in mind.
"Do not attempt to charge a wet iPhone; refer to the user guide for cleaning and drying instructions."
If your iPhone does get wet, you'd better have your instructions close to hand.
"Liquid damage not covered under warranty."
Now that's a zinger, isn't it? If water does get inside and things go wrong, you're on your own.
So no, the iPhone 7 isn't waterproof. It's water-resistant. And yes, there is a big difference between the two.