At the moment, these applications are available only for the Xperia Z1s and Xperia Z, and only in the US. They make use of the two handsets’ resistance to water, and of the fact that the smartphones literally know when they're underwater. How can they do that? Well, Sony relies on a detector that generates test tones (with a frequency that we, humans, cannot hear) and records sound via the two microphones of the smartphones. Spectrograms are then generated, and their different values help detect “in air and in water states.” If you’re a developer interested in making Underwater Apps, follow the source link at the end of this article for more info.
There are 6 Underwater Apps that are available for the beginning: Goldie, Photo Lab, Plantimal, Sink Sukn, Rainy-oke, and Tiny Umbrella. They don’t really have a purpose, other than providing entertainment and showing off the water-resistance capabilities of Sony’s phones. You can see all the Underwater Apps in the slideshow below, with download links included on the right side of each video. We remind you that you’ll need an Xperia Z or Xperia Z1s bought from the US to use the apps.
source: Sony / GitHub via Xperia Blog