We've a pair of buddywatcher ultrasonic pingers, they can be paired in the air but you have to have the emitters practically touching. I.e. the effective range is like an inch in the air. Underwater it's a good 30-50 metres. (And I can't hear their ping, BTW.)
The radio is of course the opposite.
So I can see why the ultrasound-only option would be suboptimal for pre-dive gear check, but seriously: just because the radio part works topside, doesn't prove the ultrasonic part will after you splash.
The radio is of course the opposite.
So I can see why the ultrasound-only option would be suboptimal for pre-dive gear check, but seriously: just because the radio part works topside, doesn't prove the ultrasonic part will after you splash.