In fact prior to this, Sony’s previous marketing efforts for its Xperia Z series showed users snapping photos underwater or the phone being dunked in water. Oddly enough despite the phones being rated IP68, Sony doesn’t want you to do that with the Xperia Z5. This was noted on the product’s official page.
Sony writes, “You should not put the device completely underwater or expose it to seawater, salt water, chlorinated water or liquids such as drinks. Abuse and improper use of device will invalidate warranty.” Sony goes on to note that exposure to droplets of rain, touching with wet fingers, or spilling drinks on the device is fine.
However taking it underwater is not OK. Apparently the IP68 rating was given based on strictly controlled experiments, as opposed to real-world testing in which Sony cannot guarantee your phone’s safety. In any case we suppose this is just Sony trying to cover all their legal bases. At the same time damaging your phone while using it underwater will void its warranty, so that’s worth taking note of.