I haven’t seen the show, but my guess is that they are voice overing the dialogue. Pressure gas & mix will probably make the divers voices pretty annoying and the sound quality of a mic’d face mask would be low to begin with. Voice over on all the in water chatter would make it less difficult to understand and give a better continuity of sound.
I've used comms on every episode i've been on, and we never did audio pickups to make up for deficiency of sound quality on the dives.