While we're certain no one would leave their phone lying in filth, the Z2 can stand up to 30 minutes in freshwater. We left it for a total of 15 minutes swimming in diluted coffee.
The only issue with jumping in the water with your Z2 is the pressure may be too hard for you to continue recording. We tried doing this a couple of times and only managed to capture jumping-into-water footage by tilting the phone at a 45 degree angle, otherwise recording just cuts off. But otherwise, the phone is able to record underwater video by hitting the side record-button on the phone.