You should note that the phone is still only water resistant and not waterproof. The ingress protection, IP68, is resistant to dust and resistant to submersion up to a maximum depth of 1.5m underwater for up to thirty minutes. Remember though, that the tests determining the level of resistance are done with fresh water under laboratory conditions.
While it may survive the occasional photo in those conditions, I would advise against using it for the purpose of taking underwater pictures in salt water.
Taken from Quick Start Guide.
For additional information regarding the ratings, you can refer here.
There are some pretty good reasonably priced waterproof cameras on the market. We've taken ours in the pool and a few lakes and oceans with rather good photos. I understand it's an extra item to carry, however, it's designed for use underwater. Despite laboratory testing, a cellular phone is not.
Hope this helps