Per the above from Super Cecil, I don't profess to understand the flap situation on this Beaver, and that may have been a factor in the last, steep, tight turn which failed.
I'm wondering why they went from flying North to then turning 270deg to the Right and end up heading West into Jerusalem Bay and the accident site.
Was this a standard departure path from this location? (No, according the ALA register in the ATSB report)
Did something happen onboard which meant the right turn continued more than usual?
I'm not a seaplane pilot either, but if faced with a box canyon situation, wouldn't it be better to pull power and land on whatever water remains than to try tight 180 turn at low level?
We may never know the answers, but I, like many, would like to know as much as possible to avoid it in future. It's yet another very sad aviation event.