We destroyed my son’s iPhone SE (that we use as a Dexcom receiver) in our kayak camping trip. Since it never got in contact with liquid water, I thought it was interesting to share what unexpected circumstances can ruin an iPhone despite many precautions.
We carefully bagged the iPhone in a cellphone drybag such as this waterproof pouch. This bag allowed us to handle the phone without unbagging the phone.
https://images-na.ssl-images-amazon.com/images/I/81KVz6zLFiL.SL1200.jpg
Then we boxed the whole bag into this type of waterproof lexan box. So there were two levels of waterproofness.
http://gear-report.com/wp-content/uploads/2010/07/OP_drybox.gif
We occasionally accessed the inside bag. When we did so, we did it carefully, and never in an exposed area. Neither the box nor the bag ever got water inside. Yet, before the end of the trip, the iPhone was dead and all three of its moisture sensors had tripped.
My only explanation is that the condensation water, vaporized by the sun through the transparent box and bag, must have been enough to steam the iPhone through. One hint that makes me believe that it is a likely possibility is that we did get a temperature alert on the iPhone once, while it was in the box.
So, in my next trip, I will not use a solid transparent box, but an opaque guide bag (Watershed Aleutian Deck Bag) to keep the inner waterproof bag in.
The other thing I will do is that I will bring cheaper iPod Touch devices along 
No phones in the kayaks this time.