In Season 3, Episode 18, the spoken communication among UBRI members appears to be via some form of wireless device (most likely ultrasonic, or ultrasonic-to-surface-to-radio).
Kellar: Still hasn’t noticed us.
Hellegren: Definitely less alert than
the girls. But we must be quick.
Kellar: What’s your position?
UBRI: 20 metres from target.
Kellar: Full speed.
Kellar: About to dive directly towards you.
Here the UBRI team leader underwater appears to be talking to the people abovewater through a wireless communication unit, with them relaying their instructions the same way.
Underwater communication between UBRI commandos in this episode is gestural, or perhaps via a sign language.
In Season 3, Episode 1, the mechanism is similar. Notice how the dialogue sounds mechanical, like a radio. Also note how it cuts off sharply before and after Jason and his mother start speaking.
The receivers are probably inside the mouthpiece, much as in real life.
That said, while this is obviously supposed to be the case in-universe, the masks portrayed don't seem to be the sort that allow speech.
That said, since this is science fiction, the devices could be something like this, which doesn't require a full-face mask.