The iBubble project looks very interesting -- one of the first subs I've seen designed with a killer app in mind. How do they transmit the diver's position underwater? From a wristband that talks to the sub the way sea animals do, with low-frequency communications that travel through water well:
We have already developed its critical functionality: the positioning system’s transmitter. You can see its first prototype handled by our diver in the Technical Prototype V2 video above. How does it work? The transmitter emits a low frequency signal that the drone uses to calculate its position relative to the diver. The antenna, battery and electronics included in this prototype seem bulky, but they are design to meet a >1 km (~1 mile) range requirement. Once they are scaled back to iBubble’s 25m/80ft range, they will fit in a dive computer.