You need a full face diving mask to be able to vocalize.
You need a waterproof - pressure proof microphone installed near lips.
You need a waterproof - pressure proof second contact microphone worn attached to your throat to trigger a vox.
You need an ultrasonic transceiver, modulated by voice, with vox circuitry.
You need a waterproof - pressure proof omnidirectional piezo element transducer.
You need a circuitry to filter out bubble and breathing noises.
You need molded waterproof-depthproof jacks and plugs and canister for the electronics and battery.
You need a pool nearby for your tests.
You need a lot of time, patience and adrenaline to conquer maaaany unexpected failures. But has to be much easier today with internet for sourcing parts instead of a Thomas Register, and stamped mail communications from overseas.
I worked with the inventor of the first scuba intercom around 1972.
Today; I would attempt it by light modulation (UV?); but still need many of the above items.
I have not given too much tought to a simpler wimpy megaphone style, but not to be discarded at all.