Divers need to stay in touch with each other. Casio Logosease can help. It is a 2-way underwater radio walkie-talkie device. It has a range of up to 100 meters and can work at depths of 40 meters. How does it work? It’s not really magic but all down to ultrasonic waves. All you have to do is attach the device to your diving mask and you will be able to talk using an air regulator in your mouth. Your voice is demodulated and received by your partner via a bone conduction speaker.
The device has a 560mAh Li-ion battery and can go for 2 hours on a full charge. Not cheap but might be useful for frequent divers