Water will destroy your iPhone in no time what so ever If you own and use an iPhone in or around an environment that is predominately water based, such as the beach or on board a boat, and you do not have your phone installed in a waterproof case then you are asking for trouble.
Whilst your iPhone is water resistant to a certain degree it is by no means water proof. There are many components within your iPhone that if allowed to get wet, or subjected to humid or damp environments for prolonged periods of time, will most likely fail leaving your phone inoperable. this is also true of the touch screen, which can become distorted and difficult to use if water is allowed to get into it.
There are many waterproof iPhone cases available on the internet that range from the relatively cheap (at about $35) to the more expensive (costing over $100). Clearly the cheaper cases, actually better described as pouches, are less functional than the more expensive cases that have been designed and built to allow you to take your iPhone underwater and still operate the camera, and with a pair of waterproof headphones, the phone
So, whether your work takes you to water laden environments or your lifestyle is such that you spend much of your time surrounded by water do not take a chance with your iPhone. Do your research, take your time and select a waterproof iPhone case that has been designed to protect your iPhone whilst still offering you complete access to all the features and functions that you need.