DryCase. I went scuba diving with one off the coast of Hawaii's big island and it did exceptionally well.
It's a dry bag that has a vacuum valve so you know it's sealed before getting in the water. There is no question like with every other case. My brother-in-law just this week had his Lifeproof case fail in the pool.
It also has no fail depth. Crush depth will kill your device before the bag gives out. They had one get lost in a deep pond for one month and when it was finally found the owners pulled the device out of the bag, charged it and it turned right back on. It's fantastic.
Here's a video from when I used it a few years ago on the island. All underwater footage is while snorkeling.
The only real issue is that you can't get audio as its operating in a vacuum. I wish they made a waterproof mic accessory as they have a waterproof headphone port but they don't yet. Underwater it'd be fairly useless apart from bubble noises and clicking but above water it would be great.
I've seen that one on Amazon too. Looks pretty handy but I wonder if a housing would be better. i.e. multi-purpose. Scuba and for when I'm on the beach or booting around doing other adventure stuff it would be protected more.
One thing is for sure, the price is good. Unfortunately you have to vacuum seal it every time. I guess it's no big deal since even with a housing you'd have to take it off. At least with the bag you can still use the iPhone normally.