The Sony Xperia Z is designed to be both dust and waterproof, although a number of owners of the handset are starting to report some waterproof issues with the handset.
According to a number of reports, there appears to be an issue with the raised back panel on the handset, which is causing it to slightly separate from the cause and leading to waterproof issues with the device.
As yet there does not seem to be any sort of official word from Sony on it as yet, you can find out more details about the issue over at the Sony Mobile talk forum.
Source Xperia Blog