One of the problems with hand signals and boards lies in the fact that they can't always be clearly seen from a distance – or in murky water – plus the diver who is sending the initial message has to make sure that the recipient is already looking in their direction.

Unfortunately radio communications aren't an option, as radio waves don't travel well through the water. There are acoustic voice communications systems, but they require both divers to be using expensive transceivers.

Seeking a simpler and more affordable alternative, a team at the University of Washington developed an app that can be used on a smartphone in an underwater housing. Named AquaApp, it allows users to choose between 240 preprogrammed messages which correspond to hand signals used by divers.

In order to keep things simple, messages conveying the 20 most commonly used signals are prominently displayed for quick access. Additionally, the messages can be sorted into eight subject categories, such as those relating to environmental factors or equipment status.

Once a message has been selected, the phone's speakers send it through the water as a series of acoustic pulses. These pulses are detected by the mic of the recipient's phone, where the app converts them back into a visual on-screen message. The app also alerts the recipient to the fact that a message has been received.