YES IT IS. my IPhone 7 was submurrsed in at least 2 feet of salt water in the Beaufort River, Beaufort, SC on July 8, 2017.
This was as the result of a boating mishap whereby my boat began taking on water and had to be run up on a sand bar to keep it from sinking. My phone fell in the water at the rear of the boat and remained UNDER WATER for at least 2-3 hours BEFORE I found it. The only protection it had was a soft case with a screen protector.
To to be honest, I was very surprised. I fact, I had already planned to call the insurance company to order a replacement phone. Later in the day, when I heard it accepting a text message, I was astounded.
Now, maybe I was just lucky but it is THE FIRST CELL PHONE IN 25 Years that DID NOT FAIL AFTER GETTING EVEN A LITTLE WET, MUCH LESS BEING UNDER 2 FEET OF SALT WATER FOR PROBABLY 3 HOYRS AND STILL WORKED PERFECTLY !!