All the years I've played this game, many sessions in this zone, and I've never been in the underwater area where this mob, and a few other nameds, spawn. One benefit of doing hunter achievements, it forces you to discover parts of zones you've never seen before.
TO access this area, as soon as you zone into CT and you enter the middle green area, turn left and go up top and there's a door and some tunnels that lead down to this area.