The German made ‘Buddy Watcher’ is a wrist or upper arm mounted system that uses ultrasound technology to produce a vibrating and visual signal in a second unit.

Each green and black plastic unit has two large buttons and a panel of LED lights. (The buttons look chunky enough that even a 5mm wet glove would be able to operate this unit). One button sets up and pairs the devices, whilst the second button ‘calls’ your buddy. Meanwhile the receiving device vibrates and the red LED's light up.

This is a far more discrete and direct way of alterting your buddy (than using a horn or rattle). The unit is powered by two rechargeable batteries, which are sealed within the unit. They are charged via a USB connector.

The manufacturer states that the working range between a pair of units is approximately 20m (65ft), and the Buddy Watcher has a useful depth rating of 40m (131ft).