When they're suiting up and installing the oxygen tubes he says "oxygen flow on".
It's definitely not objective.
I checked Biosector01 which has a transcript, here's what it has (cutting out the parts showing what's onscreen. Also apparently the guy doing all the systems checks as they descend is Axonn, huh... the more you know):
Axonn: Deep sea descent in progress.
Axonn: Thirty one, thirty two.
Axonn: Thirty three thousand.
Hahli: Stay on site.
Axonn: Prepare for impact.
Axonn: Release fireblade.
Axonn: Oxygen flow on.
Axonn: Three. Two. One.
Axonn: Destination reached.
Axonn: Disarm hatch. And release Cordak Blasters.
Axonn: Mission objective: Retrieve Mask of Life.
Kongu: I'm ready.
Axonn: Time is limited.
Jaller: There seems to be no sign of life.
Hahli: Whoa. What's that?
Jaller: Unite in battle formation.
Nuparu: Let's get them.
Matoro: I've got this one.
Kongu: Off you go, creep.
Nuparu: Guys, are you okay? Report back.
Hewkii: Holy Gadunka! Watch out, he's armed.
Hewkii: Jaller, Nuparu, what's your status?
Jaller: I've found the mask!
Hydraxon: I believe that mask belongs to me.
Teridax: That mask is mine.
Jaller: Guys, I need backup.
Matoro: I'll take it from here.