iPhone 7 is water resistant not waterproof and I hope you understand the difference between waterproof and water-resistant well if you are confused by the terms I can explain you.
Water resistant is the lowest level of protection from water if you have a device in which it is labeled that it is water resistant it simply means that the device is been designed in a way which helps in restricting the water enter in the device. However, water-resistant devices have some extent of protection.
Waterproof simply means a device is fully protected from water no matter to what extent it gets to drop down it will work same as it used to be. Till now no company officially launched a waterproof device neither even the Apple Corporation.
So, iPhone 7 is water resistant and you don’t need to worry if it gets a splash of water or if you dropped it down in a bucket. As it's water resistant to an extent of 30 min in depth of 1 meter.
Apple Corporation has officially stated that damaged cause to a device because of water will not be covered under the 1-year warranty.