The Age article quotes a fisherman saying he saw a plane flying at about 100ft heading straight towards ominous looking clouds. Of course, it could have been a SAR plane after the event.
One thing that I think hasn't been mentioned here yet is the condition of misjudging height above water when in clear air and with no solid horizontal references. We often hear of this when the water is glassy smooth, but the illusion could have been strong enough to captivate four pilots on that day.