Mr Parsons said he heard over the VHF radio a female was “having a seizure”.

She was serving in the Women’s Royal Naval Service in Portsmouth, passing orders to France via her VHF radio.

The military trained her in the then-revolutionary technology of VHF radio, in which only one party could transmit at a time.

“The mariner’s use of a VHF radio for their distress call was critical to our ability to quickly locate them and coordinate an effective response.”

He said it was not necessary for guides to ask participants about their sea swimming confidence, to carry a VHF radio or to work with an assistant.