2 years ago they had to reduce the Busselton ironman to a bi-athlon due to presence of lasrge marine animals. As people may heaps to enetr and its one of the few qualifying events for Hawaii people were very upset...(despite still being alive)
Knowing that I'm even happier I swam in the enclosure. It's a pretty big area (as you probably know.), I imagine it would be fine for shorter triathlon swims. I can imagine people weren't happy but as you say at least they lived!
And there was I thinking the enclosure was to keep out manatees...