Sony points out that its waterproof Xperia devices achieved an IP68 rating under laboratory conditions in standby mode, and adds that these devices were not tested for the kind of rigor of a swimming pool, or being splashed around at fast speed.
Notice in the above statement how Sony talks about achieving the IP68 certification under “laboratory conditions”. This language is echoed on the same support page:In the video clip below, Sony markets its Xpera Z3 as a fully waterproof smartphone. You can see how someone dives into a pool with the phone and takes underwater pictures. This was a pretty big part of Sony's marketing campaign but if we had to guess it appears it caused quite a lot of RMAs...
“Sony devices that are tested for their waterproof abilities are placed gently inside a container filled with tap water and lowered to a depth of 1.5 metres. After 30 minutes in the container, the device is gently taken out and its functions and features are tested.” [Emphasis ours]
You can see what Sony is doing there. Sony says that the handsets were not tested for the kind of rigor of a swimming pool, for example, where the Xperia device is potentially being splashed around at fast speed. A comment from a Sony Mobile support rep makes a similar point:
“Moving or operating the device while it is submerged is not tested during the laboratory tests. There are also many environmental factors which we could not assess (e.g. water movement or water pressure changes during the movement), if a device is used underwater. Therefore we recommend not submerge our Xperia Z5 in water.”
Source: Xperia Blog