Underwater communications

I am trying to build underwater comms for communication with a diver(hard wired)
I have waterproof 5watt speaker/mic connected to 50m 3 core cable (extension lead). on the end of the lead I have put a 4pole aux plug so I can connect to a mobile phone. Picture a normal headset with a 50m lead.
I have made the system work with a short cable but with a long cable I do not have signal strength so I think I have to amplify the signal. Quality of sound is not so important so a class D could do the job maybe? Is it possible to amplify a signal both directions.
Hope this question makes sense as I am much better at being underwater than understanding electronics
Thanks for any help

Hi Chris,

I’m a diver myself! You will have problems transmitting a signal reliably over that distance with a standard 2.5m aux plug. If you want to transmit an analog audio signal that far you need to use a shielded XLR cable or something. Also I don’t see how you will transmit and receive down the same cable. It would only make sense to have an amplifier at the surface, since you probably wouldn’t have batteries to power the amp at the bottom.

Good luck with your project!